[GH-212] Fix dropping foreign key multiple times with test

This issue is created automatically through a Github pull request on behalf of sdepablos:

Url: https://github.com/doctrine/dbal/pull/212

Message:

In some cases the Comparator class returns multiple drops for the same foreign key.
Specifically, in case you have two tables, A & B, with A having a foreign key FK
referencing B, if you drop table B, the resulting diff shows this FK twice,
once on the diff->orphanedForeignKeys array as we're deleting B, and another on
the diff->changedTables array as table A is also being modified. As a result of this you
get the DROP FOREIGN KEY instruction twice in the final SQL.