Changed 'stat' json key from 'mime-type' to 'content-type'
This is for two reasons. The first is for consistency with the keys from
the 'manifest' endpoint. The other is that this key matches the name of
the HTTP header field definition;
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html sec 14.17.
Merge pull request #2 from strootman/ChangeStatJsonKey

Changed 'stat' json key from 'mime-type' to 'content-type'