Fixed SQL syntax for postgresql

activerecord/test/cases/serialized_attribute_test.rb

@@ -96,7 +96,7 @@ def test_json_db_null
 
     # On 4.0, re-saving a row with a database NULL will turn that into a JSON
     # "null"
-    assert_equal 1, Topic.where('content = "null"').count
+    assert_equal 1, Topic.where("content = 'null'").count
   end
 
   def test_serialized_attribute_declared_in_subclass