Case insensitive header tests?

[hurrymaplelad]

Http treats headers as case-insensitive. Node normalizes headers to lowercase to make them easy to work with. I'd expect the assertion interface to be as liberal as the http spec, so I could test

    expect(response).to.have.header('Cache-Control', 'no-cache')

Would you take a PR to lowercase header names before retrieving them from node?

[keithamus]

Once again @hurrymaplelad, great idea, would happily merge a PR of that nature!