revert Default timestamps to non-null

Commit 3dbedd2 added NOT NULL constraints to timestamps.
Commit fcef728 started to revert this, but was incomplete.
With this commit, 3dbedd2 should be fully reverted and
timestamps will no longer default to NOT NULL.

Conflicts:
	activerecord/test/cases/migration/change_schema_test.rb
	activerecord/test/cases/migration/column_attributes_test.rb