Use the API URL character encoding for XMLHttpRequest

Unfortunately forcing UTF-8 is not web compatible.

This also cleans up some other steps in the open() method in the process.

Tests: https://hg.mozilla.org/releases/mozilla-beta/rev/fd6134823ff9.

Fixes #159.

xhr.bs

@@ -446,33 +446,25 @@ and
 methods, when invoked, must run these steps:
 
 <ol>
- <li><p>If <a>context object</a>'s
- <a>relevant settings object</a> has a
- <a>responsible document</a> and it is <em>not</em>
- <a>fully active</a>,
- <a>throw</a> an <code>InvalidStateError</code> exception.
-
- <li><p>If <var>method</var> is not a
- <a for=/>method</a>,
- <a>throw</a> a
- <code>SyntaxError</code> exception.
-
- <li><p>If <var>method</var> is a <a>forbidden method</a>,
- <a>throw</a> a
- <code>SecurityError</code> exception.
-
- <li><p><a for=method>Normalize</a>
- <var>method</var>.
-
- <li><p>Let <var>parsedURL</var> be the result of
- <a lt="URL parser">parsing</a> <var>url</var> with
- <a>context object</a>'s
- <a>relevant settings object</a>'s
- <a>API base URL</a>.
+ <li><p>Let <var>settingsObject</var> be <a>context object</a>'s <a>relevant settings object</a>.
+
+ <li><p>If <var>settingsObject</var> has a <a>responsible document</a> and it is <em>not</em>
+ <a>fully active</a>, then <a>throw</a> an "{{InvalidStateError!!exception}}" {{DOMException}}.
+
+ <li><p>If <var>method</var> is not a <a for=/>method</a>, then <a>throw</a> a
+ "{{SyntaxError!!exception}}" {{DOMException}}.
 
- <li><p>If <var>parsedURL</var> is failure,
- <a>throw</a> a
- <code>SyntaxError</code> exception.
+ <li><p>If <var>method</var> is a <a>forbidden method</a>, then <a>throw</a> a
+ "{{SecurityError!!exception}}" {{DOMException}}.
+
+ <li><p><a for=method>Normalize</a> <var>method</var>.
+
+ <li><p>Let <var>parsedURL</var> be the result of <a lt="URL parser">parsing</a> <var>url</var> with
+ <var>settingsObject</var>'s <a for="environment settings object">API base URL</a> and
+ <var>settingsObject</var>'s <a for="environment settings object">API URL character encoding</a>.
+
+ <li><p>If <var>parsedURL</var> is failure, then <a>throw</a> a "{{SyntaxError!!exception}}"
+ {{DOMException}}.
 
  <li>
   <p>If the <var>async</var> argument is omitted, set <var>async</var> to true, and set
@@ -495,11 +487,10 @@ methods, when invoked, must run these steps:
    <var>password</var>.
   </ol>
 
- <li><p>If <var>async</var> is false, <a>current global object</a> is a {{Window}}
- object, and the {{XMLHttpRequest/timeout!!attribute}} attribute value is not zero
- or the {{XMLHttpRequest/responseType}} attribute value is not the
- empty string, then <a>throw</a> an
- <code>InvalidAccessError</code> exception.
+ <li><p>If <var>async</var> is false, <a>current global object</a> is a {{Window}} object, and the
+ {{XMLHttpRequest/timeout!!attribute}} attribute value is not zero or the
+ {{XMLHttpRequest/responseType}} attribute value is not the empty string, then <a>throw</a> an
+ "{{InvalidAccessError!!exception}}" {{DOMException}}.
 
  <li>
   <p><a for=fetch lt=terminated>Terminate</a> the ongoing fetch operated by the