Adding unit test assertion assert_same_as

I came across your gem today when I was looking for something like this for my tests. Since I'm using unit tests instead of RSpec I was missing assertions that would give me a useful failure message. I'm currently relearning Ruby/Rails (I haven't used it in 5 years), so as an exercise I implemented what I needed on top of your gem. This appears to work for me, but please review the code, I might be making some beginner's mistakes.

Note that I copied a bunch of code from spec_helper.rb into test_helper.rb. That should be extracted into a module shared between the two, but I'm not sure where to best put it. If you have ideas, I'd be happy to try to fix this.