xml.result, xml.test:
  Adding test case.
item_xmlfunc.cc:
  - adding "size" member into MY_XPATH_FLT struct,
  to pass parent's context size when iterating
  in a predicate. Previously, temporaty context
  size was calculated instead, which is always 1.
  As a result, things like last() and count() 
  didn't work fine.
  - adding iteration into Item_func_xpath_elementbyindex:
  similar to Item_func_xpath_predicate.
  This is to make things like last() and count()
  work inside square brackets.