fixes foreman_openscap#57 - update digest when replacing file

[shlomizadok]

fixes https://github.com/OpenSCAP/foreman_openscap/issues/57

[ares]

I don't think this is the right way, caching of digest is actually a good idea. We should rather check before validation that the scap_file has changed and recalculate the digest in that case. Something like before_validation_on_update :redigest, :if => { |c| c.scap_file_changed? }

[shlomizadok]

@ares - agreed. Changed to before_validation. As before_validation(:on_update) is Rails4, I have added a check to the :if, scap_content.persisted?. Let me know what you think.

[ares]

not tested but visually looks good to me :+1:

[isimluk]

Merged as 8f533e9b6ae6876fc9bafa74a44079096db544c7. Thank You!