Make res.set() work like express when passed an object

[gmaclennan]

The Express res.set() accepts an object as an argument to set headers. This PR should copy that functionality from Express.

[howardabrams]

Interesting addition. Can you write a test to validate this change?

Thanks a lot!

[gmaclennan]

Added a test. This does, however, introduce a breaking change. Previously mockResponse.header() was an alias of either getHeader or setHeader depending on the amount of passed parameters, but this change sets it as an alias to mockResponse.set(), which mimics express. I'm not sure where mockResponse.header() would be used by anyone, because as it was it was not an Express method or a standard node.http method. Mocking Express seems to make more sense.

[howardabrams]

I think you are correct. Shortcut aliases like this seems like a bad idea now.