Output of repos.getArchiveLink not usefull

When requesting a repo's archive link, the output of `getArchiveLink` is useless as it misses the location header that refers to the actual archive link: 

`{ 'x-ratelimit-limit': '5000', 'x-ratelimit-remaining': '4969' }`

Not sure what the correct output should be, but output-wise my commit aligns it with the API documentation for [get-archive-link](http://developer.github.com/v3/repos/contents/#get-archive-link); all its output is in the headers, but for usability sake we could add it in the main response.

Totally open for discussion here, though.