Message1512

The first patch also introduced this bug. 

>>> u'%s' % 's'
's'

It seems PyUnicode inherits str___mod__ from PyString directly and the old implementation used createInstance which is overridded in PyUnicode. 

Need to add a test for this one. It probably would be a good idea to add tests also for unicode join and replace (e.g. "u''.join([])") to make sure similar issues don't appear there.

A fix for this could be overriding str__mod__ in PyUnicode so that the implementation in PyString is called first and then its results casted to PyUnicode.