Adding support for spree_xhr_post, spree_xhr_put, spree_xhr_delete

In the spree ControllerRequests module there is currently testing support helpers for spree_get, spree_post, spree_put and spree_delete.  There is also the xhr equivalent of spree_xhr_get - however, there is no equivalent for post, put or delete.  I added support for these 3 HTTP methods and extracted the common functionality into a private method.