Allow used body replacement in Request constructor

Currently, the following code snippet replaces `request`'s body as one
would expect:

  let request = new Request(url, { method: "POST", body: "foo" })
  request = new Request(request, { body: "bar" })

But this snippet throws a TypeError early in Request's constructor:

  let request = new Request(url, { method: "POST", body: "foo" })
  await request.text()  // disturb the body
  request = new Request(request, { body: "bar" })  // throws

This commit's changes allows the latter code snippet to work like the
first one.

Fixes whatwg#674.

fetch.bs

@@ -5088,8 +5088,9 @@ constructor must run these steps:
   <p>Otherwise (<var>input</var> is a {{Request}} object), run these substeps:
 
   <ol>
-   <li><p>If <var>input</var> is <a for=Body>disturbed</a> or <a for=Body>locked</a>, then
-   <a>throw</a> a <code>TypeError</code>.
+   <li><p>If <var>input</var> is <a for=Body>disturbed</a> or <a for=Body>locked</a> and
+   <var>init</var>'s <code>body</code> member is not present, then <a>throw</a> a
+   <code>TypeError</code>.
 
    <li><p>Set <var>request</var> to <var>input</var>'s
    <a for=Request>request</a>.
@@ -5351,7 +5352,8 @@ constructor must run these steps:
  <!-- Any steps after this must not throw. -->
 
  <li>
-  <p>If <var>inputBody</var> is non-null, then run these substeps:
+  <p>If <var>inputBody</var> is non-null and <var>inputBody</var> is <var>body</var>, then run these
+  substeps:
 
   <ol>
    <li>
@@ -5366,10 +5368,9 @@ constructor must run these steps:
     <p class="note no-backref">This makes <var>inputBody</var>'s <a for=body>stream</a>
     <a for=ReadableStream>locked</a> and <a for=ReadableStream>disturbed</a> immediately.
 
-   <li><p>If <var>inputBody</var> is <var>body</var>, then set <var>body</var> to a new
-   <a for=/>body</a> whose <a for=body>stream</a> is <var>rs</var>, whose <a for=body>source</a>
-   is <var>inputBody</var>'s <a for=body>source</a> and whose <a for=body>total bytes</a> is
-   <var>inputBody</var>'s <a for=body>total bytes</a>.
+   <li><p>Set <var>body</var> to a new <a for=/>body</a> whose <a for=body>stream</a> is
+   <var>rs</var>, whose <a for=body>source</a> is <var>inputBody</var>'s <a for=body>source</a> and
+   whose <a for=body>total bytes</a> is <var>inputBody</var>'s <a for=body>total bytes</a>.
   </ol>
 
  <li><p>Set <var>r</var>'s <a for=Request>request</a>'s <a for=request>body</a> to <var>body</var>.
@@ -6251,6 +6252,7 @@ Glenn Maynard,
 Graham Klyne,
 Hal Lockhart,
 Hallvord R. M. Steen,
+Harris Hancock,
 Henri Sivonen,
 Henry Story,
 Hiroshige Hayashizaki,