Fix comparison of URLs and request method.

* String#casecmp returns -1/0/1, not true/false
* use request.request_method instead of request.method, which contains the
  intended method, not the physically used one (PATCH vs. POST)
* call url_for on the url, and compare with both full url and path; this fixes
  the case where a symbol or array is specified as url in signed_form_for
Add failing spec for method check.
Stylistic changes.
Add changelog entry for #6
Merge pull request #7 from erichmenge/schuetzm-fix_url_comparison

Fix comparison of URLs and request method.