Empty fields don't get deleted when at least one meta-key field is filled in

[kanduvisla]

If I understand correctly, the correct behaviour of this extension is to delete meta-key-fields which aren't filled in. This works if none of the fields are filled in.

However... when at least one field is filled in, alle empty fields get stored as well. I believe this is incorrect behaviour.

[kanduvisla]

I made you a nice pull request which fixes this issue, and makes the automatic deletion of empty keys optional:

#5