Set request header's guard if no-cors

Fixes #1074

Author: jakearchibald

fetch.bs

@@ -6384,6 +6384,10 @@ constructor steps are:
  <a for=Headers>header list</a> is <var>request</var>'s <a for=request>header list</a>, and
  <a for=Headers>guard</a> is "<code>request</code>".
 
+ <li><p>If <a>this</a>'s <a for=Request>request</a>'s <a for=request>mode</a> is
+ "<code>no-cors</code>", then set <a>this</a>'s <a for=Request>headers</a>'s
+ <a for=Headers>guard</a> to "<code>request-no-cors</code>".
+
  <li>
   <p>If <var>init</var> <a for=map>is not empty</a>, then: