The widely used DownloadEngine.getResourceCacheEntry(),
did not handle the case: !cacheEnabled && doDownload,
which it should.
It is not recommended to encapsulate all 'fetch resource' calls
in the caller with 'isCacheEnabled', since it leads to
duplicate and confusing code.
In case of !cacheEnabled && doDownload,
DownloadEngine.getResourceCacheEntry() has to use
a temporary cache entry where the resource is downloaded into
and the caller can return the temporary file handle (File instance).