Delete request-body-headers for redirects altering the request method

Tests: web-platform-tests/wpt#20596.

Fixes #609.

fetch.bs

@@ -890,6 +890,16 @@ from APIs using <a for=/>fetch</a> that allow control over
  <li>`<code>Set-Cookie2</code>`
 </ul>
 
+<p>A <dfn export>request-body-header name</dfn> is a <a for=/>header</a> <a for=header>name</a> that
+is a <a>byte-case-insensitive</a> match for one of:
+
+<ul class=brief>
+ <li>`<code>Content-Encoding</code>`
+ <li>`<code>Content-Language</code>`
+ <li>`<code>Content-Location</code>`
+ <li>`<code>Content-Type</code>`
+</ul>
+
 <hr>
 
 <p>To <dfn export lt="extract header values|extracting header values">extract header values</dfn>
@@ -4028,11 +4038,19 @@ optional <i>CORS-preflight flag</i>, run these steps:
    <li><p><var>actualResponse</var>'s <a for=response>status</a> is <code>301</code> or
    <code>302</code> and <var>request</var>'s <a for=request>method</a> is `<code>POST</code>`
    <li><p><var>actualResponse</var>'s <a for=response>status</a> is <code>303</code> and
-   <var>request</var>'s <a for=request>method</a> is not `<code>HEAD</code>`
+   <var>request</var>'s <a for=request>method</a> is not `<code>GET</code>` or `<code>HEAD</code>`
   </ul>
 
-  <p>then set <var>request</var>'s <a for=request>method</a> to `<code>GET</code>` and
-  <var>request</var>'s <a for=request>body</a> to null.
+  <p>then:
+
+  <ol>
+   <li><p>Set <var>request</var>'s <a for=request>method</a> to `<code>GET</code>` and
+   <var>request</var>'s <a for=request>body</a> to null.
+
+   <li><p><a for="list">For each</a> <var>headerName</var> of <a>request-body-header name</a>,
+   <a for="header list">delete</a> <var>headerName</var> from <var>request</var>'s
+   <a for=request>header list</a>.
+  </ol>
 
  <li>
   <p>If <var>request</var>'s <a for=request>body</a> is non-null, then set <var>request</var>'s