[xml] problem with xmlSetProp in rev 1.366

[Rob Richards <rrichards ctindustries net>]

Sorry I didn't catch this change earlier, but I think the change made to 
not add an attribute with a : in its name when no namespace is found is 
incorrect. The change I am referring to is rev 1.366 of tree.c:
   /*
   * If we get a QName and the prefix has no namespace-
   * binding in scope, then this is an error.
   * TODO: Previously this falled-back to non-ns handling.
   *   Should we revert this?
   */
   return(NULL);

A colon is a legal character for an attribute name. Although not where 
this issue cropped up and why I didn't catch it before, take DOM for 
example. For DOM level 1 where there is no namespace support, the 
attribute should be added to the document with the colon in its name.

I think this should be reverted.

Rob