Small change to association name sigularization

This is to avoid having to define irregular pluralizations for both snake-case and camel-case version of a multi-word expression.

Say you hava a model called `PluralIrregular` with an associated table `plurales_irregulares` (which isn't that bad in spanish).

I'll have to put two lines in inflections.rb to make this work:

    inflect.irregular 'plural_irregular', 'plurales_irregulares'
    inflect.irregular 'PluralIrregular', 'PluralesIrregulares'

This can be painful with many irregular plurals. Of course, you can change the pluralization rules if all your class names will be in spanish, but I found myself having to use quite a lot of irregular plurals for various reasons.