Only use Content-Type when non-null to be consistent with XMLHttpRequest

Fixes w3c/ServiceWorker#363

Overview.html

@@ -7,7 +7,7 @@
 
 <p><a class="logo" href="//www.whatwg.org/"><img alt="WHATWG" height="100" src="//resources.whatwg.org/logo-fetch.svg" width="100"></a>
 <h1 id="cors">Fetch</h1>
-<h2 class="no-num no-toc" id="living-standard-—-last-updated-5-august-2014">Living Standard — Last Updated 5 August 2014</h2>
+<h2 class="no-num no-toc" id="living-standard-—-last-updated-6-august-2014">Living Standard — Last Updated 6 August 2014</h2>
 
 <dl>
  <dt>This Version:
@@ -33,7 +33,7 @@ <h2 class="no-num no-toc" id="living-standard-—-last-updated-5-august-2014">Li
 <p class="copyright"><a href="http://creativecommons.org/publicdomain/zero/1.0/" rel="license"><img alt="CC0" src="http://i.creativecommons.org/p/zero/1.0/80x15.png"></a>
 To the extent possible under law, the editor has waived all copyright and
 related or neighboring rights to this work. In addition, as of
-5 August 2014, the editor has made this specification available
+6 August 2014, the editor has made this specification available
 under the
 <a href="http://www.openwebfoundation.org/legal/the-owf-1-0-agreements/owfa-1-0" rel="license">Open Web Foundation Agreement Version 1.0</a>,
 which is available at
@@ -2355,7 +2355,7 @@ <h3 id="body-stream-concept"><span class="secno">5.2 </span>Body stream concept<
 <ol>
  <li><p>Let <var title="">stream</var> be an empty byte stream.
 
- <li><p>Let <var title="">Content-Type</var> be an empty byte sequence.
+ <li><p>Let <var title="">Content-Type</var> be null.
 
  <li>
   <p>Switch on <var title="">object</var>'s type:
@@ -2766,7 +2766,8 @@ <h3 id="request-class"><span class="secno">5.3 </span>Request class</h3>
    <li><p>Set <var title="">r</var>'s <a href="#concept-request-request" title="concept-Request-request">request</a>'s
    <a href="#concept-request-body" title="concept-request-body">body</a> to <var title="">stream</var>.
 
-   <li><p>If <var title="">r</var>'s <a href="#concept-request-request" title="concept-Request-request">request</a>'s
+   <li><p>If <var title="">Content-Type</var> is non-null and <var title="">r</var>'s
+   <a href="#concept-request-request" title="concept-Request-request">request</a>'s
    <a href="#concept-request-header-list" title="concept-request-header-list">header list</a> contains no
    <a href="#concept-header" title="concept-header">header</a> <a href="#concept-header-name" title="concept-header-name">named</a>
    `<code title="">Content-Type</code>`, <a href="#concept-headers-append" title="concept-Headers-append">append</a>
@@ -2926,7 +2927,8 @@ <h3 id="response-class"><span class="secno">5.4 </span>Response class</h3>
    <li><p>Set <var title="">r</var>'s <a href="#concept-response-response" title="concept-Response-response">response</a>'s
    <a href="#concept-response-body" title="concept-response-body">body</a> to <var title="">stream</var>.
 
-   <li><p>If <var title="">r</var>'s <a href="#concept-response-response" title="concept-Response-response">response</a>'s
+   <li><p>If <var title="">Content-Type</var> is non-null and <var title="">r</var>'s
+   <a href="#concept-response-response" title="concept-Response-response">response</a>'s
    <a href="#concept-response-header-list" title="concept-response-header-list">header list</a> contains no
    <a href="#concept-header" title="concept-header">header</a> <a href="#concept-header-name" title="concept-header-name">named</a>
    `<code title="">Content-Type</code>`, <a href="#concept-header-list-append" title="concept-header-list-append">append</a>

Overview.src.html

@@ -2311,7 +2311,7 @@ <h3>Body stream concept</h3>
 <ol>
  <li><p>Let <var title>stream</var> be an empty byte stream.
 
- <li><p>Let <var title>Content-Type</var> be an empty byte sequence.
+ <li><p>Let <var title>Content-Type</var> be null.
 
  <li>
   <p>Switch on <var title>object</var>'s type:
@@ -2722,7 +2722,8 @@ <h3>Request class</h3>
    <li><p>Set <var title>r</var>'s <span title=concept-Request-request>request</span>'s
    <span title=concept-request-body>body</span> to <var title>stream</var>.
 
-   <li><p>If <var title>r</var>'s <span title=concept-Request-request>request</span>'s
+   <li><p>If <var title>Content-Type</var> is non-null and <var title>r</var>'s
+   <span title=concept-Request-request>request</span>'s
    <span title=concept-request-header-list>header list</span> contains no
    <span title=concept-header>header</span> <span title=concept-header-name>named</span>
    `<code title>Content-Type</code>`, <span title=concept-Headers-append>append</span>
@@ -2882,7 +2883,8 @@ <h3>Response class</h3>
    <li><p>Set <var title>r</var>'s <span title=concept-Response-response>response</span>'s
    <span title=concept-response-body>body</span> to <var title>stream</var>.
 
-   <li><p>If <var title>r</var>'s <span title=concept-Response-response>response</span>'s
+   <li><p>If <var title>Content-Type</var> is non-null and <var title>r</var>'s
+   <span title=concept-Response-response>response</span>'s
    <span title=concept-response-header-list>header list</span> contains no
    <span title=concept-header>header</span> <span title=concept-header-name>named</span>
    `<code title>Content-Type</code>`, <span title=concept-header-list-append>append</span>