create a way to track whether a resource property was set null or was not set

[spacedevin]

currently there is no way to see if we set null to a property. this will be useful in omitting these fields when inserting into the database to use the default values for the table.

we already have 2 separate arrays: Resource::fields() and Resource::properties(). dont null fill properties, and instead loop over fields() when we need all fields.

[spacedevin]

Triaged for Tipsy 2.0 (#57)

Still open. Resource::load() still blank-fills missing fields; there is no distinct “unset vs null” tracking for omitting columns on insert.