Add OPTIONS support to File and MockRequest.

Also update the specs for same.

RFC 2616, section 9.2 specifies OPTIONS requests.  Not much effort to
support it for this case; there's already an ALLOWED_VERBS cosntant in
Rack::File, and the spec describes the body of the request as optional.
Minor fix to Rack::File OPTIONS for Lint.
Merge pull request #614 from pete/master

Add OPTIONS support to File and MockRequest.