Fix overrideMimeType() again

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.

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

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 +1300,24 @@ 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. <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><dfn id=final-charset>Final charset</dfn> is the return value of these steps:
+
+<ol>
+ <li><p>If <a>override MIME type</a>'s <a for="MIME type">parameters</a>["<code>charset</code>"] is
+ not nothing, then return it.
+
+ <li><p>If <a>response MIME type</a>'s <a for="MIME type">parameters</a>["<code>charset</code>"] is
+ not nothing, then return it.
+
+ <li><p>Return null.
+</ol>
+
+<p class=note>The above steps intentionally do not use <a>final MIME type</a> as it would yield the
+wrong result.
 
 <hr>
 
@@ -1474,28 +1480,26 @@ 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 <var>mimeTypeRecord</var> to a new
+ <a>MIME type</a> whose <a for="MIME type">type</a> is "<code>application</code>" and
+ <a for="MIME type">subtype</a> is "<code>octet-stream</code>".
 </ol>