Actually define Blob-to-ReadableStream conversion

Together with #1505 this fixes #1504.

fetch.bs

@@ -6544,10 +6544,19 @@ typedef (ReadableStream or XMLHttpRequestBodyInit) BodyInit;</pre>
 steps:
 
 <ol>
- <li><p>Let <var>stream</var> be <var>object</var> if <var>object</var> is a {{ReadableStream}}
- object. Otherwise, let <var>stream</var> be a <a>new</a> {{ReadableStream}}, and
+ <li><p>Let <var>stream</var> be null.
+
+ <li><p>If <var>object</var> is a {{ReadableStream}} object, then set <var>stream</var> to
+ <var>object</var>.
+
+ <li><p>Otherwise, if <var>object</var> is a {{Blob}} object, set <var>stream</var> to the result of
+ running <var>object</var>'s <a for=Blob>get stream</a>.
+
+ <li><p>Otherwise, set <var>stream</var> to a <a>new</a> {{ReadableStream}} object, and
  <a for=ReadableStream>set up</a> <var>stream</var>.
 
+ <li><p><a for=/>Assert</a>: <var>stream</var> is a {{ReadableStream}} object.
+
  <li><p>Let <var>action</var> be null.
 
  <li><p>Let <var>source</var> be null.
@@ -6562,8 +6571,6 @@ steps:
   <dl class=switch>
    <dt>{{Blob}}
    <dd>
-    <p>Set <var>action</var> to this step: read <var>object</var>.
-
     <p>Set <var>source</var> to <var>object</var>.
 
     <p>Set <var>length</var> to <var>object</var>'s {{Blob/size}}.