Performs the action requested by the given XHR and returns an object to be applied
on to the XHR (containing `status`, `responseText`, etc.). For the most part,
this is delegated to `doMethod` methods on this class, such as `doGet`.

@param {Ext.ux.ajax.SimXhr} xhr The simulated XMLHttpRequest instance.
@returns {Object} The response properties to add to the XMLHttpRequest.
function (xhr) {
            var me = this,
                ret = {},
                method = 'do' + Ext.String.capitalize(xhr.method.toLowerCase()), // doGet
                fn = me[method];

            if (fn) {
                ret = fn.call(me, me.getCtx(xhr.method, xhr.url, xhr));
            } else {
                ret = { status: 405, statusText: 'Method Not Allowed' };
            }

            return ret;
        }