Fixed a bug that RecordFilterMixin does not work with stream with multiple records

"format_stream" passes tag variable to "filter_record" without deep
copying.
It works fine when there is only 1 record in stream.
But If the stream have n records, filter_record filters tag n times.
Fixed code formatting issues

- add space between , and tag
- use dup to clone string object
Merge pull request #109 from tomohisaota/fix-mixin

Fixed a bug that RecordFilterMixin does not work with stream