Add PUT request support

fix empty request body when PUT request :
Until now :

Client side : (where `xhr` is `<polymer-ajax>` element)

        this.$.xhr.xhrArgs = {
          headers: {
            "Content-Type": "application/json"
          },
          body: JSON.stringify({name:"bob"})
        };

Server side : (express sample)

		app.put("/buddies/:id", function(req, res){
			var updatedBuddy = req.body
			res.send(JSON.stringify(updatedBuddy))
		});

`updatedBuddy` equals `{}` after `this.$.xhr.go()` with `this.$.xhr.method ="PUT"`

with fix, we've got `updatedBuddy` equals `{name:"bob"}`