associations#find should use scoped(construct_scope).find except with custom finder_sql

There's a lot of logic duplication in associations.

When a find(options) is triggered on an association, the following things currently happen explicitly in the find method:

• If it's got :finder_sql, a separate branch of code is taken.
• If not (99% of the time):
  • The generated association finder_sql is merged with options[:conditions]
  • The reflection's :order is merged with options[:order]
  • Other options are reverse_merged on top of the reflection's options
  • @reflection.klass.find(*args) is finally called

This sort of merging of conditions, order, and more is already accomplished by ActiveRecord scope merging. Associations are really just named_scopes that change the main table anyway.

Association already have the construct_scope method, which can construct a complete finder scope representing that association. Using @reflection.klass.scoped(construct_scope[:find]), all of that redundant merge logic can be finally deleted out of the #find method.