KW args fix

@myronmarston support for https://github.com/rspec/rspec-support/pull/89
Not necessarily better, but this could be:

``` ruby
::RSpec::Support.register_matcher_definition(&method(:is_a_matcher?))
```

My mental model of ruby suggests this might be faster (as I think there'll be one less call frame in there).  I haven't benchmarked it though.