Fix: Remove MathML out if used as input

* For some reason it causes problems if the input
  MathML is used in the output and contains special chars.
* Since the output MathML is not really needed in the output
  the mml field is set to '' and src field
  is set to the static value 'mathml' in that case.

Bug: 62921
Change-Id: I014da024e20cd080bec6c4f8c9a6da73cc38675e