Access an instance's connection via its class, rather than via #connection

Had a problem today where we had a `belongs_to :connection` on our model. Hadn't occurred to me that this might be a problem, but when we ran our tests we were getting an obscure `undefined method substitute_at for nil:NilClass` exception whenever an instance of this class was being destroyed.

Discovered the root cause was this method, which was attempting to call `connection` on the instance, which in turn [delegates to the class](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/core.rb#L328). This seems like a pretty safe change to make, I can't think of any possible repercussions. I wasn't sure if this warranted a regression test, but I'd be happy to provide one if it's required.