Add `test_decode_number` as there is a test that encode number. Check this _iteration_[1] for a solution that pass the tests without checking for alphanumeric in decode.
Add `test_encode_decode`. Having done this in the first place, would had highlighted the need of checking for alphanumeric in decoding.
[1] http://exercism.io/submissions/c7c9448e6a674dd6a11ea35dc32e29a4
