added put,delete method, added X-Request-With header

I think headers should be Set instead of Seq.
Ajax request should include X-Requested-With header like other javascript frameworks do such as JQuery.
In Play! framework, if we setup the CSRF filter, we need to add either CSRF token to the request
or have X-Requested-With in the header. Otherwise the filter will return 403  
[http://www.playframework.com/documentation/2.2.x/ScalaCsrf]
I also added 2 more methods PUT and DELETE because they are pretty common in REST.
What do you think?
If the order does not matter, would it make sense to store this as a `Map[String, String]`?