handle duplicate keys

Handle duplicate keys correctly, e.g. the same way that the request module does it (should fix #1)
We broke backwards compatibility in xhr recently anyway, so I wouldn't be so careful to return the value for single occuring headers, it's hard to handle that way. I think you can create an array for a single value too.
Less code, more consistency for the end-user.

Also, that's what you get from npm `request` module