[web-api] Correct MIME type check.

Fixes #1138.

document/web-api/index.bs

@@ -76,6 +76,7 @@ url:https://w3c.github.io/webappsec-secure-contexts/#environment-settings-object
 
 <pre class='link-defaults'>
 spec:ecma-262; type:exception; for:ECMAScript; text:TypeError
+spec:fetch; type:dfn; text:get
 spec:webidl; type:dfn; text:resolve
 </pre>
 
@@ -118,7 +119,9 @@ Note: This algorithm accepts a {{Response}} object, or a
     1. Let |returnValue| be [=a new promise=]
     1. [=Upon fulfillment=] of |source| with value |unwrappedSource|:
         1. Let |response| be |unwrappedSource|'s [=Response/response=].
-        1. Let |mimeType| be the result of [=extracting a MIME type=] from |response|'s [=response/header list=].
+        1. Let |mimeType| be the result of [=getting=] `` `Content-Type` `` from |response|'s [=response/header list=].
+        1. Remove all [=HTTP tab or space=] from the start and end of |mimeType|.
+        1. [=ASCII lowercase=] |mimeType|.
         1. If |mimeType| is not `` `application/wasm` ``, reject |returnValue| with a {{TypeError}} and abort these substeps.
 
             Note: extra parameters are not allowed, including the empty `` `application/wasm;` ``.