PostgreSQL Timestamps always map to `:datetime`.

The PG Adapter should use `:datetime` consistently instead of mapping
mispellings to `:timestamp`.

See #14513
Merge pull request #14859 from jefflai2/datetime_type

PostgreSQL Timestamps always map to `:datetime`.
reset `@arel` when modifying a Relation in place.

/cc @tenderlove
No need for trailing slash on migration path.

Causes a double // in Dir.glob that breaks Ruby 2.2-trunk.
Not really a bug, but not relevant to this test either.

Originally added in ed21f0c50270139ddb6993acfdaea4586ffd09a3
move AR length validation tests into separate test-case.

Conflicts:
	activerecord/test/cases/validations/association_validation_test.rb