Re-enable foriegn key tests on MySQL

This reverts commit e84799d, e31104c and e6ca8e2
Fix mysql/mysql2 failing with FK constraint errors

One of the author fixture we have ("david") references an author address by ID.
Since we disable FK checks when inserting fixtures, this is all fine until we
try to update it, at which point MySQL would complain about the missing row
referenced by the `author_address_id`.

[Godfrey Chan, Matthew Draper]