Explicit catchall Accept and Content-Type headers.

Without an `Accept` header field, Facebook returns JSON responses with
content-type `text/javascript`. As of HTTParty 0.12.0, content types of
`text/javascript` are parsed as plain text instead of JSON,
causing parsing of their response bodies to break.
Merge pull request #147 from shiau/js-explicit-accept-header

Explicit catchall Accept and Content-Type headers.