Bugfix for HMI FITS and fits.py.
If waveunit is empty string because wavelength_comment and waveunit_comment were None, return None.
Continuing bugfix PR.
1. Added resampled_hmi.fits.
2. Incremented the length of entries in test_tables:test_dir_recursive tests by 1 due to the new fits file.
Added derdon's minor change.
Merge pull request #952 from VaticanCameos/HMI-FITS-Bugfix

[Review]: [Bugfix]: HMI FITS and fits.py.