Define XMLHttpRequestBodyInit (BodyInit without ReadableStream)

See whatwg/xhr#277.

fetch.bs

@@ -5480,7 +5480,9 @@ method, when invoked, must run these steps:
 <h3 id=body-mixin>Body mixin</h3>
 
 <pre class=idl>
-typedef (Blob or BufferSource or FormData or URLSearchParams or ReadableStream or USVString) BodyInit;</pre>
+typedef (Blob or BufferSource or FormData or URLSearchParams or USVString) XMLHttpRequestBodyInit;
+
+typedef (ReadableStream or XMLHttpRequestBodyInit) BodyInit;</pre>
 
 <p>To <dfn for=BodyInit export>safely extract</dfn> a <a for=/>body</a> and a
 `<code>Content-Type</code>` <a for=header>value</a> from <var>object</var>, run these steps:
@@ -7173,6 +7175,7 @@ Gary Blackwood,
 Gavin Carothers,
 Glenn Maynard,
 Graham Klyne,
+Gregory Terzian,
 Hal Lockhart,
 Hallvord R. M. Steen,
 Harris Hancock,