[postgres] improve #insert_sql (uses primary_key, supports all AR versions)
[postgres] improve #insert_sql (uses primary_key, supports all AR versions)

keeguon/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,keeguon/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,keeguon/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter