Add failing test case for #8460

Add a test case to ensure that fractional second updates are detected.
Revert "Merge pull request #6986 from kennyj/fix_6975"

This reverts commit 8905c1fb496641c3cdb7b3b816ae6d3d4b2c2b73.

Closes #8460

Conflicts:
	activerecord/test/cases/dirty_test.rb