Fixed AssociationCollection#<< resulting in unexpected values in @target when :uniq => true

Signed-off-by: NMichael Koziarski <michael@koziarski.com>

activerecord/lib/active_record/associations/association_collection.rb

@@ -344,7 +344,7 @@ def add_record_to_target_with_callbacks(record)
           callback(:before_add, record)
           yield(record) if block_given?
           @target ||= [] unless loaded?
-          @target << record
+          @target << record unless @reflection.options[:uniq] && @target.include?(record)
           callback(:after_add, record)
           record
         end

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -299,6 +299,17 @@ def test_uniq_before_the_fact
     assert_equal 3, projects(:active_record, :reload).developers.size
   end
 
+  def test_uniq_option_prevents_duplicate_push
+    project = projects(:active_record)
+    project.developers << developers(:jamis)
+    project.developers << developers(:david)
+    assert_equal 3, project.developers.size
+
+    project.developers << developers(:david)
+    project.developers << developers(:jamis)
+    assert_equal 3, project.developers.size
+  end
+
   def test_deleting
     david = Developer.find(1)
     active_record = Project.find(1)