Issue 18: Test case for doubling data

ref https://github.com/orbitjs/orbit.js/issues/18

This test case acts a bit weird. I tried to simulate 2 calls of find. So I waited for the first one to finish, to call it again (with a different callback). Interestingly, the tests for the first call already fail, because the size == 4. However, this only happens because of the second find. If I remove the second call to find, the test passes. Did i missed something?