Changed an explicit "\"" to expression.externalNameQuoteCharacter() so that the quoting of foreign keys works for all database types regardless of how they do quoting of attributes.

Thanks to Dave (one of them... I forget which, but he's a hell of a lumberjack) and Aaron Rosenzweig for finding the hard coded quotes that affected mysql foreign key.
Thanks also to MySQL for being out there enough to break this so we could find it.