Content-Type header can be capitalized

Hi,

   I was trying to mock a POST request with a body encoded with the content type "application/x-www-form-urlencoded" and found that the "Content-Type" header entry could be either capitalized or not.

   Example: [The Hipchat API](https://www.hipchat.com/docs/api/method/rooms/message)

   This fix make it work in both cases.

Hope it helps

Thank you

Aurélien

P.S: Build fix in another PR: https://github.com/flatiron/nock/pull/109