AssociationCollection #first and #last cache.

Sometimes when testing or in irb, I seem to forget that the following does not work:

first = firm.clients.first
firm.clients.each { ... } # loads the target and thus the instance is gone
firm.clients.first.object_id == first.object_id # => false

This patch solves this.

Note that the same queries as normal are executed, just the instance in the collection result is replaced.