Fix behavior of handling BC era dates.

BC era year is (astronomical year + 1) and starts from 1 BC.
Mime::PNG is already defined.
Merge pull request #15521 from edogawaconan/fix_bc_postgres_master

Fix BC year handling in postgres
Merge pull request #15529 from zuhao/refactor_actionpack_mime_type_test

Make sure Mime::Type is not altered after tests.