Added flag has_primary_key_set to Model

Now we know whether the primary key is defined or not
and we do not allow to make update/remove if the key is not defined.

And when doing insert/update we can put NULL if child models don't have
the primary key set (fields with has_foreign_key set to true).

Now in after_select() we should also set has_primary_key_set flag
or just call get_last_sequence_for_primary_key instead of get_last_sequence.

fixed: added prefix +00 when serializing PT::Date to PostgreSQL (time zone)
(for a column with a time zone there was a wrong value saved)

src/baseexpression.cpp

@@ -118,6 +118,7 @@ void BaseExpression::dump_additional_info(Model & model)
 	if( model.model_env && model.model_env->dump_mode )
 	{
 		field(L"model_save_mode", model.save_mode, false, false, false, model.model_env);
+		field(L"has_primary_key_set", model.has_primary_key_set, false, false, false, model.model_env);
 	}
 }