Add last_updated_by feature in issues index api
Related with [#6375 Last updated by column in issue list]. This feature is very useful.
This patch just adds this feature to issue's index api.
/issues.xml returns like following.
<last_updated_by id="1" name="foo bar"/>
Tested with version 3.4.3.
Version 3.4.3 and trunk revision 17570, the target files were the same.