Change HTTP date formats from UTC to GMT to conform to section 3.3 of HTTP spec document RFC2616 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3)

Addresses issue https://github.com/square/okhttp/issues/337
Fix up HttpResponseCacheTest.setIfModifiedSince test to work with GMT timezone format in if-modified-since header
Merge pull request #339 from rupertbates/master

Change HTTP date formats from UTC to GMT to conform to section 3.3 of HT...