Added unittest test_melt_nokey() to check cases when key needs to be assumed as fields not specified in variables argument

Regression eliminated in itermelt() where key argument is not specified

 Rather than checking `if not key`, need to check `if key is None` to allow for fields named {0, 0L, 0.0, 0j, ''}
 (See: https://docs.python.org/2/library/stdtypes.html#truth-value-testing)