moved serialization of attribute text data (xmlSerializeContent) into a

* tree.c, include/libxml/tree.h: moved serialization of
  attribute text data (xmlSerializeContent) into a separate
  routine (xmlSerializeTxtContent) so it can be used by xmlwriter.c
* xmlwriter.c: changed handling of attribute string to use the
  routine above (fixed bug 131548)
4 files changed