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)

samples/type.h

@@ -5,7 +5,7 @@
  */
 
 /*
- * Copyright (c) 2019, Tomasz Sowa
+ * Copyright (c) 2019-2021, Tomasz Sowa
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -79,7 +79,7 @@ public:
 
 	void after_insert()
 	{
-		get_last_sequence(L"public.types_id_seq", id);
+		get_last_sequence_for_primary_key(L"public.types_id_seq", id);
 	}