Add more tests for mysql 5.6 microseconds + Fix tests

Two separate things in this PR:

First commit, is to add a unit test to make sure the `quote` method is working as expected.

Second:
When running `persistence_test`, multiple times, 2 tests were failing randomly, after nailing down the issue seems like the problem was happening because we were comparing `prev_month.to_i` , which has a `usec`, and if the DB didnt save that `usec` we could have round issues. I am not sure if thats the proper fix or not. please let me know what you guys think.

review @rafaelfranca