Fix overrideMimeType() again

This makes a number of changes:

* Integrates with the new MIME Sniffing infrastructure.
* Reset override MIME type when open() is invoked.
* Actually make overriding the charset parameter work (setting override MIME type to the "MIME type portion" (now essence) as done previously would always erase it).
* Use "get an encoding" (previously undefined) to convert from a label to an encoding.

Tests: web-platform-tests/wpt#8449.

Fixes #157.

xhr.bs

@@ -495,6 +495,7 @@ methods, when invoked, must run these steps:
    <a>network error</a>.
    <li><p>Set <a>received bytes</a> to the empty byte sequence.
    <li><p>Set <a>response object</a> to null.
+   <li><p>Set <a>override MIME type</a> to null.
   </ul>
 
  <li>
@@ -1299,19 +1300,32 @@ transfer-encoding: chunked</code></pre>
  <li><p>Return <var>mimeType</var>.
 </ol>
 
-<p>The <dfn id=override-mime-type>override MIME type</dfn> is initially null and can get a value if
-{{overrideMimeType()}} is invoked. <dfn id=final-mime-type>Final MIME type</dfn> is the
+<p>The <dfn id=override-mime-type>override MIME type</dfn> is initially null and can get a value
+when {{overrideMimeType()}} is invoked. The <dfn id=final-mime-type>final MIME type</dfn> is the
 <a>override MIME type</a> unless that is null in which case it is the <a>response MIME type</a>.
 
-<p>The <dfn id=response-charset>response charset</dfn> is the value of
-the <code>charset</code> parameter of the `<code>Content-Type</code>` header
-or null if there was no `<code>charset</code>` parameter or the header could
-not be parsed or was omitted. The
-<dfn id=override-charset>override charset</dfn> is initially null and
-can get a value if <a><code>overrideMimeType()</code></a> is invoked.
-<dfn id=final-charset>Final charset</dfn> is the
-<a>override charset</a> unless
-that is null in which case it is the <a>response charset</a>.
+<p>The <dfn id=final-charset>final charset</dfn> is the return value of these steps:
+
+<ol>
+ <li><p>Let <var>label</var> be null.
+
+ <li><p>If <a>response MIME type</a>'s <a for="MIME type">parameters</a>["<code>charset</code>"]
+ <a for=map>exists</a>, then set <var>label</var> to it.
+
+ <li><p>If <a>override MIME type</a>'s <a for="MIME type">parameters</a>["<code>charset</code>"]
+ <a for=map>exists</a>, then set <var>label</var> to it.
+
+ <li><p>If <var>label</var> is null, then return null.
+
+ <li><p>Let <var>encoding</var> be the result of <a>getting an encoding</a> from <var>label</var>.
+
+ <li><p>If <var>encoding</var> is failure, then return null.
+
+ <li><p>Return <var>encoding</var>.
+</ol>
+
+<p class=note>The above steps intentionally do not use the <a>final MIME type</a> as it would yield
+the wrong result.
 
 <hr>
 
@@ -1336,7 +1350,7 @@ or null). Unless stated otherwise it is null.
 
 <ol>
  <li><p>Set <a>response object</a> to a new {{Blob}} object representing <a>received bytes</a> with
- {{Blob/type}} <a>final MIME type</a>.
+ {{Blob/type}} set to the <a>final MIME type</a>.
 
  <li><p>Return <a>response object</a>.
 </ol>
@@ -1348,11 +1362,11 @@ or null). Unless stated otherwise it is null.
  <li><p>If <a>response</a>'s
  <a for=response>body</a> is null, then return null.
 
- <li><p>If <a>final MIME type</a> is not an <a>HTML MIME type</a> or an <a>XML MIME type</a>, then
- return null.
+ <li><p>If the <a>final MIME type</a> is not an <a>HTML MIME type</a> or an <a>XML MIME type</a>,
+ then return null.
 
  <li>
-  <p>If {{XMLHttpRequest/responseType}} is the empty string and <a>final MIME type</a> is an
+  <p>If {{XMLHttpRequest/responseType}} is the empty string and the <a>final MIME type</a> is an
   <a>HTML MIME type</a>, then return null.
 
   <p class=note>This is restricted to
@@ -1361,7 +1375,7 @@ or null). Unless stated otherwise it is null.
   content.
 
  <li>
-  <p>If <a>final MIME type</a> is an <a>HTML MIME type</a>, then run these substeps:
+  <p>If the <a>final MIME type</a> is an <a>HTML MIME type</a>, then:
 
   <ol>
    <li><p>Let <var>charset</var> be the <a>final charset</a>.
@@ -1406,7 +1420,7 @@ or null). Unless stated otherwise it is null.
  <a for=Document>encoding</a> to
  <var>charset</var>.
 
- <li><p>Set <var>document</var>'s <a>content type</a> to <a>final MIME type</a>.
+ <li><p>Set <var>document</var>'s <a>content type</a> to the <a>final MIME type</a>.
 
  <li><p>Set <var>document</var>'s
  <a for=Document>URL</a> to
@@ -1445,7 +1459,7 @@ or null). Unless stated otherwise it is null.
  <li><p>Let <var>charset</var> be the <a>final charset</a>.
 
  <li>
-  <p>If {{XMLHttpRequest/responseType}} is the empty string, <var>charset</var> is null, and
+  <p>If {{XMLHttpRequest/responseType}} is the empty string, <var>charset</var> is null, and the
   <a>final MIME type</a> is an <a>XML MIME type</a>, then use the rules set forth in the XML
   specifications to determine the encoding. Let <var>charset</var> be the determined encoding.
   [[!XML]] [[!XMLNS]]
@@ -1474,28 +1488,25 @@ resources using <a>utf-8</a>.
 <dl class=domintro>
  <dt><code><var>client</var> . <a method for=XMLHttpRequest>overrideMimeType(<var>mime</var>)</a></code>
  <dd>
-  <p>Acts as if the `<code>Content-Type</code>` header for <a>response</a> is <var>mime</var>.
+  <p>Acts as if the `<code>Content-Type</code>` header value for <a>response</a> is <var>mime</var>.
+  (It does not actually change the header though.)
 
   <p>Throws an "{{InvalidStateError!!exception}}" {{DOMException}} if <a>state</a> is <i>loading</i>
   or <i>done</i>.
 </dl>
 
-<p>The
-<dfn method for=XMLHttpRequest><code>overrideMimeType(<var>mime</var>)</code></dfn>
-method must run these steps:
+<p>The <dfn method for=XMLHttpRequest><code>overrideMimeType(<var>mime</var>)</code></dfn> method,
+when invoked, must run these steps:
 
 <ol>
  <li><p>If <a>state</a> is <i>loading</i> or <i>done</i>, then <a>throw</a> an
  "{{InvalidStateError!!exception}}" {{DOMException}}.
 
- <li><p>Set <a>override MIME type</a> to `<code>application/octet-stream</code>`.
-
- <li><p>If <var>mime</var> is a <a>parsable MIME type</a>, then set <a>override MIME type</a> to its
- <a>MIME type portion</a>.
- <!-- XXX Ignore string to byte sequence conversion issues until some point in the future -->
+ <li><p>Set <a>override MIME type</a> to the result of <a lt="parse a MIME type">parsing</a>
+ <var>mime</var>.
 
- <li><p>If <a>override MIME type</a> has a `<code>charset</code>` parameter, then set
- <a>override charset</a> to its value.
+ <li><p>If <a>override MIME type</a> is failure, then set <a>override MIME type</a> to
+ <code>application/octet-stream</code>.
 </ol>