No NullReferenceException when passing null to Table.Set

[bplu4t2f]

Passing null as value parameter in any of the Table.Set overloads causes a NullReferenceException in Table.PerformTableSet. I propose either a proper ArgumentNullException, or giving null a reasonable meaning. Considering the similarity to setting the value to nil (instead of null) it might be reasonable to assume that the intent of the user in this case is to remove the value from the table.

[LimpingNinja]

It's a little late, not certain if we should be checkin IsNil() here as well to be clear on the assumed intent. I may be wrong though, I probably should sleep. We do need an appropriate test case added to this PR as well if you can.