Specify identity encoding for range requests

Tests: web-platform-tests/wpt#11291.

Fixes #747.

Author: jakearchibald

fetch.bs

@@ -3652,8 +3652,19 @@ Range Requests</cite>. [[HTTP-RANGE]] However, this is not widely supported by b
     </ol>
 
    <li>
-    <p>Modify <var>httpRequest</var>'s
-    <a for=request>header list</a> per HTTP.
+    <p>If <var>httpRequest</var>'s <a for=request>header list</a> <a for="header list">contains</a>
+    `<code>Range</code>`, then <a for="header list">append</a>
+    `<code>Accept-Encoding</code>`/`<code>identity</code>` to <var>httpRequest</var>'s
+    <a for=request>header list</a>.
+
+    <p class="note no-backref"><a href="https://jakearchibald.github.io/accept-encoding-range-test/">Many servers</a>
+    mistakenly ignore `<code>Range</code>` headers if a non-identity encoding is accepted.
+
+   <li>
+    <p>Modify <var>httpRequest</var>'s <a for=request>header list</a> per HTTP. Do not
+    <a for="header list">append</a> a given <a>header</a> if <var>httpRequest</var>'s
+    <a for=request>header list</a> <a for="header list">contains</a> that <a>header</a>'s
+    <a for=header>name</a>.
 
     <p class="note no-backref">It would be great if we could make this more normative
     somehow. At this point <a for=/>headers</a> such as