Added content_encoding attribute to Rackspace storage

Added content_encoding attribute to Rackspace storage.

Amazon storage already has it and according to [this](http://docs.rackspace.com/files/api/v1/cf-devguide/content/Enabling_File_Compression_with_the_Content-Encoding_Header-d1e2198.html) Rackspace also allow setting the content-encoding.