fix inverse_of association in block of new child

This fixes inconsistency when building children of association
which has inverse_of set properly.

When creating new association object with a block:

    parent.association.build do |child|
      child.parent.equal?(parent) # false
    end

So the block the `child.parent` did not point to the same object.
But when the object is created it points to same instance:

    child = parent.association.build
    child.parent.equal?(parent) # true

1 files changed, 1 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/associations/association.rb b/activerecord/lib/active_record/associations/association.rb
index 4c4b0f08e5..6578a9096b 100644
--- a/activerecord/lib/active_record/associations/association.rb
+++ b/activerecord/lib/active_record/associations/association.rb
@@ -235,6 +235,7 @@ module ActiveRecord
             skip_assign = [reflection.foreign_key, reflection.type].compact
             attributes = create_scope.except(*(record.changed - skip_assign))
             record.assign_attributes(attributes)
+            set_inverse_instance(record)
           end
         end
     end