[FIX] partial fix of #24

- fix all direct string writing in MzMLHandler<MapType>::writeHeader_
  with writeXMLEscape
[TEST] add test which checks that the produced file can read again by OpenMS
[FIX] last fix of mzML files
Merge pull request #955 from hroest/fix/xml_escape

[FIX] xml escape in mzML handler