Fix schema comparison with FK that contain quoted column names.

In case `QuoteStrategy` decides to quote column names for FK, schema comparator (and in effect - schema tool), will be stuck in an `ALTER TABLE * DROP FOREIGN KEY` loop, because it doesn't recognise old indexes with matching column names.

The problem lies inside Comparator, which relied on column names that might or might not be quoted (depending on platform, schema and dbal settings). The comparison must be performed on **unquoted** column names both ways, as it is already performed for normal indexes, table names and columns names.

Tests will come shortly.