Support request headers

[elliotttf]

Some APIs use the accept header to negotiate the correct version to serve but it doesn't appear to be possible to set this during instantiation. Furthermore, nodeunit-httpclient assumes that if the data for a request is an object the content-type header should be application/json and not necessarily whatever the Accept header is set to. This PR allows the client to be instantiated with an appropriate content-type request header and prevents overriding the header with application/json if it has already been set.

[powmedia]

I don't really use this module any more but if you can merge in the latest changes from master (from your other PR) then I'll merge this in. Thanks!