Delete fixture and get removed fixture function for later restoration ~8

[justinbmeyer]

From: canjs/canjs#810

Something like:

can.fixture("services/thing", function(){ ... CODE A ... });

// in a test, remove default fixture and provide your own
var oldFixture = can.fixture("services/thing", null);
can.fixture("services/thing", function(){ ... CODE B ... });
// run test
TEST

// restore old fixture
can.fixture("services/thing", oldFixture)