Fix handling of #instance_eval in AssociationProxy

Hi all,

In the current stable, the AssociationProxy#method_missing object
doesn't handle calls to #instance_eval properly, because of the way it
handles the block. This is a small, tested patch to fix the problem:

https://rails.lighthouseapp.com/projects/8994/tickets/3412-patch-correctly-handle-instance_eval-in-association-proxies

I'd love to get this reviewed and committed into core - thanks!

Mat