Force decoding of gzip'ed files.

Ruby 2.0.0's `net/http` handles GZip encoded content properly, while 1.9.3's doesn't. 

When run under 2.0.0 files downloaded from both the original version of `fetch` and one mirroring this PR's version are identical, but under 1.9.3 the original version of fetch returns raw zlib output. Please review https://gist.github.com/rjackson/6326033 for a demonstration. 

This resolves #243.