Explicit catchall Accept and Content-Type headers.

Without an 'Accept' header field, Facebook returns JSON responses with
content-type 'text/javascript'. Due to HTTParty 0.12.0's new treatment
of 'text/javascript' content types as plain text, as opposed to JSON,
parsing these responses breaks any request where Mogli originall
expected a JSON response.
Update tests to expect an explicit Accept header field.

As of HTTParty 0.12.0, content types of 'application/javascript' are
parsed as plain text instead of JSON. Facebook is returning
'application/javascript' responses when no Accept header field is
passed, causing parsing of their response bodies to break.