Bug 1120715 - Part 2: Don't use OPEN_TRUNCATE when reopening a fake synthesized cache entry; r=mayhemer,jdm

This shows up when intercepting the reload cache type channels, since
the cache storage will reopen the cache entry that has been used to fill
up the synthesized response otherwise because of the LOAD_BYPASS_CACHE
flag.

netwerk/protocol/http/nsHttpChannel.cpp

@@ -3035,6 +3035,10 @@ nsHttpChannel::OpenCacheEntry(bool isHttps)
     } else {
         if (mInterceptCache == INTERCEPTED) {
             cacheEntryOpenFlags |= nsICacheStorage::OPEN_INTERCEPTED;
+            // Clear OPEN_TRUNCATE for the fake cache entry, since otherwise
+            // cache storage will close the current entry which breaks the
+            // response synthesis.
+            cacheEntryOpenFlags &= ~nsICacheStorage::OPEN_TRUNCATE;
             DebugOnly<bool> exists;
             MOZ_ASSERT(NS_SUCCEEDED(cacheStorage->Exists(openURI, extension, &exists)) && exists,
                        "The entry must exist in the cache after we create it here");