Make String serialize empty string to empty string instead of null

This makes the following True:
``` String().serialize(node, u'') == u'' ```

Issue #72
we arent going to release a 3-dot float-fear colander
Merge branch 'master' of github.com:Pylons/colander
Merge branch 'string_type_serialize_empty_string' of git://github.com/tranminhh/colander into tranminhh-string_type_serialize_empty_string
add change note