Patch for has_one :through associations

I have opened a ticket for a small patch that fixes two small errors
in has_one :through associations:

1) An unassigned has_one :through association should return nil when
referenced. Instead, it returns an empty array. This is due to the
call to #reset_target! on AssociationCollection, which is the most
derived class that defines that method in the inheritance hierarchy
for HasOneThroughAssociation.

2) Assigning to a has_one :through association does not correctly
assign to the target of the association. This is due to the way the
target gets set in the attribute writer method. The attribute writer
creates the join association, then delegates to the attribute reader,
with an explicit reload, to recreate the has_one :through association
and save it on the instance. However, after calling the attribute
reader, the attribute writer overwrites the instance variable with an
out of date association object.

The patch includes tests for each of these behaviors, along with the
fixes.

http://rails.lighthouseapp.com/projects/8994/tickets/895-has_one-through-errors