Fix exception raised in method "id=" when inserting new record and postgres db migration failure (issue #196) 

Below exception was raised.
lib/composite_primary_keys/composite_arrays.rb:19:in `parse': Unsupported type: 0 (ArgumentError)

It's because new_id 0 was returned in below code.

 74       def create_record(attribute_names = nil)$
 75         record_timestamps!$
 76         attribute_names ||= keys_for_partial_write$
 77         attributes_values = arel_attributes_with_values_for_create(attribute_names)$
 78 
 79         new_id = self.class.unscoped.insert attributes_values$
 80         self.id = new_id if self.class.primary_key$