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: thatcher@a46fe6b#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.