Provide a way to use can-fixture without XHR wrapping

[matthewp]

There are several downsides to wrapping XHR, mostly surrounding the large API surface that is required to correctly do it. It would be good for users to have a way to use can-fixture without the XHR shim.

I propose a method that, when called, will check the fixture store first and if it not found, fallback to making a normal HTTP request. I propose something like:

var fixture = require("can-fixture");

fixture.request({
  method: 'GET',
  url: '/foo/bar'
}).then(function(response){

});

We need to define the shape of that object based on what can-fixture supports. One thing that is unclear to me is if can-fixture is an XHR-only library or if we want to keep it generic enough that fetch could be used instead. If so the shape of the object might need to be more generic to support both.

An adapter could be built for can-connect that uses this method.