fix column linking

Hi, I think I found a bug in coopy-ruby.  If I understand correctly, the code is checking whether each column index is >= 0, assuming the index would be -1 if not found, but the hash will actually return a nil object if the key is not found in either of the hashes.  I added a spec that fails with the current version of the code and passes afterwards.  If I've misunderstood something about the code, my apologies, and please let me know.

Thanks for building this library, and thanks for your feedback on the pull request!