Use object equality rather == to check for singleton DB_DEFAULT.

Might address #198

lib/schema_plus/active_record/attribute.rb

@@ -9,7 +9,7 @@ def original_value_with_schema_plus
         # prevent attempts to cast DB_DEFAULT to the attributes type.
         # We want to keep it as DB_DEFAULT so that we can handle it when
         # generating the sql.
-        return DB_DEFAULT if value_before_type_cast == DB_DEFAULT
+        return DB_DEFAULT if value_before_type_cast.equal? DB_DEFAULT
         original_value_without_schema_plus
       end