Add a DB env var for testing against postgres, mysql
Drop quoted columns in SET clauses. Fixes #64, Reverts #60.

There is no One Quoting Syntax To Rule Them All. For a SET statement,
postgres and mysql seems to disagree about how a column name should be
quoted if it is quoted at all. Here, we go back to un-quoted column
names. Though columns named "order" and other reserved words will cause
errors, the solution works in all other cases.