Fixed a spacing issue involving sizing commands and operators

This addresses issue https://github.com/Khan/KaTeX/issues/30.
I think that this isn't something that should be tested with code, but should probably use a Huxley test instead. Generally, it's not helpful to verify that the code we write produces the structure we tell it to produce, because the test will just ensure that we produce the code tells it to, and not that the produced output is useful. The Huxley tests ensure that the code we write produces something that looks correct.

Did you ever get huxley tests to work?