Add explicit flattening option to Downloader:Http

With the change in 0.16.3, which disabled flattening for zip files
some pods seem to break. While the rationale for the flattening
change is a good one, it would be nice to reenable flattening support
explicitly in the source configuration for your pod.

references https://github.com/CocoaPods/CocoaPods/issues/812
Merge branch 'master' of git://github.com/synyx/CocoaPods into synyx-master
Rewrite a spec that expects a certain error to be raised.