Update counter cache when pushing into association

This commit fixes a regression bug (issue #3891) in which counter_cache columns were not being updated correctly when newly created records were being pushed into an assocation. EG:

    # this was fine
    @post.comment.create!

    # this was fine
    @comment = Comment.first
    @post.comments << @comment

    # this would not update counters
    @post.comments << Comment.create!