Special characters in question fields in Moodle XML export didn't properly escaped for plugins using extra_question_fields

I discovered bug in my universal XML export code from MDL-20157 (it's affects only 3d party plugins for now). Extra question fields could contain HTML special chars and they are not escaped properly.

Should be easy to fix using <![CDATA[ ]]> tag if such characters existed, adapting solution used by XML format writetext function. I'm going to commit patch in the near time.