Added contentType/Encoding to HttpResponse's Entity.

Clients should expect that HttpResponse Entity's contentType and contentEncoding are not null when the headers are present.  Added unit tests to cover various combinations of Content-Type and Content-Encoding along with transparent gzip handling.

I moved the 5 responseBody/entity lines up in OkApacheClient so that BasicHeader objects were not created again behind the scenes while setting the entity's contentType and contentEncoding.