GMOD-281 calling a non-final variant of AbstractHttpClient.execute

allowing use of adapted non-AbstractHttpClient backend client (e.g.
CachingHttpClient)
Response is handled by the handler passed to the 'execute' method.

The response entity is consumed once this method has returned, it can't
be parsed then.
Merge pull request #16 from jageay/master

GMOD-281 calling a non-final variant of AbstractHttpClient.execute