Fix method signature to work properly on 1.8.7.

When passing a matcher as an argument, it checks
`could_contain_kw_args?`, which was defined on 1.9+
but not on 1.8.7.
Expose required/optional kw args from method signature.

This is needed for rspec/rspec-mocks#786.
Changelog.
Merge pull request #116 from rspec/fix-method-signature-1.8.7

Fix method signature 1.8.7