Add #description method to Header and HTTP Status Code matchers

I'm using some of the matchers in this way on my tests:

it { expect(response).to be_ok }
it { expect(response).to be_in_json }

And RSPEC shows a big warning message in the failures saying that the matcher didn't had a #description method, nor I had a String in my test.
I just added the #description methods to Header and HTTP Status Code matchers becauses I thought that those are the ones that would be used in the way showed above.