Handle the constructor case where priority is set in init but internal priority is not.

Author: pmeenan

fetch.bs

@@ -7474,9 +7474,15 @@ constructor steps are:
  <li><p>If <var>init</var>["{{RequestInit/signal}}"] <a for=map>exists</a>, then set
  <var>signal</var> to it.
 
- <li><p>If <var>init</var>["{{RequestInit/priority}}"] <a for=map>exists</a> and
- <var>request</var>'s <a for=request>internal priority</a> is not null, update
- <var>request</var>'s <a for=request>internal priority</a> appropriately.
+ <li>
+  <p>If <var>init</var>["{{RequestInit/priority}}"] <a for=map>exists</a>, then:
+  <ol>
+   <li><p>If <var>request</var>'s <a for=request>internal priority</a> is not null, update
+   <var>request</var>'s <a for=request>internal priority</a> appropriately.
+
+   <li><p>Otherwise, set <var>request</var>'s <a for=request>priority</a> to the value of
+   <var>init</var>["{{RequestInit/priority}}"].
+  </ol>
 
  <li><p>Set <a>this</a>'s <a for=Request>request</a> to <var>request</var>.