Allow blank embargo release date

We are using embargo in Curate, but users currently cannot
manually switch an object from embargoed status. Once a date
is set, it cannot be removed because embargo_release_date
doesn't accept nil values.
Add test for replacing an embargo date with a nill value
Merge pull request #177 from projecthydra/embargo-for-6-4-branch

Bring embargo fixes to 6.4 branch