Add failing test case for #8460

Add a test case to ensure that fractional second updates are detected.
Revert "Round usec when writing timestamp attribute."

This reverts commit e9d2ad395ec2ef929d74752f3d71c80674044fbe.

Closes #8460

Conflicts:
	activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
	activerecord/test/cases/dirty_test.rb