[v2] Behavior sluggable enforces index creation

[ghost]

After adding the behavior "sluggable" some migrations before

<behavior name="sluggable" />

in every new migration generated with

app/console propel:migration:diff

the sluggable index shall be dropped and created again

DROP INDEX `my_table_slug` ON `my_table`;
CREATE UNIQUE INDEX `my_table_slug` ON `my_table` (`slug`(255));