Bug 1378308 - nsHttpChannel::GetAlternativeDataType returns alt-data type when delivering data from network, r=valentin

When racing cache with network and the network wins, we don't reset mAvailableCachedAltDataType. So when the cache entry has been opened and has alt-data then nsHttpChannel::GetAlternativeDataType returns alt-data type, but we're delivering raw response (no conversion are applied) from the network.

netwerk/protocol/http/nsHttpChannel.cpp

@@ -7007,6 +7007,7 @@ nsHttpChannel::OnStartRequest(nsIRequest *request, nsISupports *ctxt)
             MOZ_ASSERT(request == mTransactionPump);
             LOG(("  First response from network\n"));
             mFirstResponseSource = RESPONSE_FROM_NETWORK;
+            mAvailableCachedAltDataType.Truncate();
         } else if (WRONG_RACING_RESPONSE_SOURCE(request)) {
             LOG(("  Early return when racing. This response not needed."));
             return NS_OK;