Migrate tests to tape

[simov]

@mikeal @nylen

I just noticed that the oauth-sign tests are migrated to tape in test-oauth The first 6 tests are the same as the first 3 here + the rsa signing for each one, added in this PR request/request#1228

My idea is to move all unit tests specific to signing out of test-oauth and put them here. That way we can concentrate on tests specific to the oauth implementation in request there. The other thing is that we'll have all tests migrated to tape here and that will allow us to enable coverage tool for example.

Let me know what do you think.