Case-insensitive check for content type before setting it

[YarnSaw]

Updated tests/test-headers.js to test for the case this PR addresses.

If the following conditions were met it would result in the content-type header always being "text/plain;charset=UTF-8":

• The method is not GET or HEAD
• Data is being sent
• When setting the content-type header, the key used is anything other than a case-sensitive "Content-Type"

I encountered it when sending a POST request with xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded');, then was surprised by the server receiving the content-type as text/plain.

[mjwwit]

Nice find! Thanks for adding the necessary test changes as well.