Accepting non xml replies to XMLHttpRequests

[emirikol]

When making an xhr request env-js croaks when receiving a non-xml response regardless of content-type headers. this can be avoided like this: https://github.com/thatcher/env-js/commit/a46fe6b1ff59f0596c162ee01e94ff256ee47bfe#diff-19

Note: that commit contains one error - checking request headers instead of response headers.

[smparkes]

If you can generate a pull request against the johnson fork, that'd be great.