Do really include the beacon Blob content type if valid.

Source/core/loader/BeaconLoader.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/loader/BeaconLoader.h"
 
 #include "core/FetchInitiatorTypeNames.h"
 #include "core/dom/DOMArrayBufferView.h"
 #include "core/dom/Document.h"
@@ skipping 129 matching lines @@
 {
     ASSERT(data);
     RefPtr<FormData> entityBody = FormData::create();
     if (data->hasBackingFile())
         entityBody->appendFile(toFile(data)->path());
     else
         entityBody->appendBlob(data->uuid(), data->blobDataHandle());
 
     request.setHTTPBody(entityBody.release());
 
-    AtomicString contentType;
     const String& blobType = data->type();
     if (!blobType.isEmpty() && isValidContentType(blobType))
-        request.setHTTPContentType(AtomicString(contentType));
+        request.setHTTPContentType(AtomicString(blobType));
 
     return true;
 }
 
 unsigned long long Beacon::beaconSize(PassRefPtr<DOMArrayBufferView> data)
 {
     return data->byteLength();
 }
 
 bool Beacon::serialize(PassRefPtr<DOMArrayBufferView> data, ResourceRequest& request, int, int&)
@@ skipping 27 matching lines @@
     request.setHTTPBody(entityBody.release());
     request.setHTTPContentType(contentType);
 
     payloadLength = entitySize;
     return true;
 }
 
 } // namespace
 
 } // namespace blink