Improve testing

Hey @ohler55,

This is my attempt to improve oj test suite. I've started this work in order to add Rails integration tests (as we discussed), but I've run into various troubles. I hope this PR is a sane way to improve test suite, from your point of view.

There are 2 tests failing, I'm not sure what cause is, so it would be great to use your help a little. Let me know what you think anyway. Check out commit message for a detailed description :sparkles: 

Addresses #152.