Bug 1217501 P4 Fill headers before setting guard when reading response from Cache. r=ehsan

dom/cache/TypeUtils.cpp

@@ -349,11 +349,15 @@ TypeUtils::ToInternalRequest(const CacheRequest& aIn)
   RefPtr<InternalHeaders> internalHeaders =
     ToInternalHeaders(aIn.headers(), aIn.headersGuard());
   ErrorResult result;
-  internalRequest->Headers()->SetGuard(aIn.headersGuard(), result);
-  MOZ_ASSERT(!result.Failed());
+
+  // Be careful to fill the headers before setting the guard in order to
+  // correctly re-create the original headers.
   internalRequest->Headers()->Fill(*internalHeaders, result);
   MOZ_ASSERT(!result.Failed());
 
+  internalRequest->Headers()->SetGuard(aIn.headersGuard(), result);
+  MOZ_ASSERT(!result.Failed());
+
   nsCOMPtr<nsIInputStream> stream = ReadStream::Create(aIn.body());
 
   internalRequest->SetBody(stream);