Follow-up: Set PG UUID PKs for non-duplicate features on UPDATE mode

[gacarrillor]

Complex subject :), let's summarize how do we handle PKs:

PRIMARY KEY	APPEND mode	UPDATE mode
Automatic PK (e.g., serial)	It lets the provider (e.g., PostgreSQL, GeoPackage, etc.) fill the value automatically	Duplicate features: It doesn't modify the value already stored Non-duplicate features: Let's the provider fill the value automatically, except for UUID PKs on PostgreSQL, where the value is set from the source layer (if given)!
Non-automatic PK	You need to provide a value for the PK in the source layer, because such value wil be set in the target layer's PK	Duplicate features: It doesn't modify the value already stored Non-duplicate features: The value is set from the source layer (it should always be given)!

New tests and test enhancements included.

Ref #42