Dates passed through body in a_request method aren't correctly compared

When dates are passed as parameters in body of the method `a_request` and are compared in method `matching_hashes?` they are in different formats, even being generated in the same way. 

This happen because when the `JSON.parse` do the conversion of a date the format is modified and the comparison in method `matching_hashes?` doesn't consider it, comparing peer-to-peer.

Example:
```ruby
it "should satisfy expectation even if json had date in the content" do
  body_hash['date'] = Date.today
  lambda {
    http_request(:post, "http://www.example.com/", :headers => {'Content-Type' => 'application/json'}, 
                       :body => body_hash.to_json)
    a_request(:post, "www.example.com").with(:body => body_hash).should have_been_made
  }.should_not raise_error
end
```

I don't know if this is the better way to go, neither if is a real problem. So, let me know if I need to change something.