Model::Field compact_attributes should not return nils; unset_attributes should be a thing

[kornypoet]

1. compact_attributes does return "nil but set" attributes. This differs from attributes.compact, which is surprising behavior.
2. unset_attributes should exist, with the current functionality of compact_attributes, and...
3. unset_attributes should trigger the firing of the defaults, because...
   • otherwise calling or not calling attributes and then compact_attributes give different behavior; this should be addressed by...
4. attribute_set? should return true for a field with a default.
   • this is somewhat controversial, but if you believe as flip does that an attribute with a default has always had that value, just lazily evaluated then the behavior is correct.

Bug atually filed by flip, so if TD disagrees he may follow up...