move call to xmlKeepBlanksDefault(0) to before the creation of an XML parser context

move call to xmlKeepBlanksDefault(0) to before the creation of an XML parser context

the value is used by the parser context; the old code called it only after the *first* parser context
was created. therefore the first XMLTree::read() call has its behavior determined by libxml2's default
for this value, rather than by our explicit choice (do not treat whitespace as a note). All subequent
read() calls will use our value.

This variable inside libxml2 is actually per-thread, which just increases the stakes for calling
xmlKeepBlanksDefault() at the right time