ixml: implement if_ixml_node~get_namespace_uri

[oblomov-dev]

What is wrong

The method is a stub, src/ixml/cl_ixml.clas.locals_imp.abap:

METHOD if_ixml_node~get_namespace_uri.
  ASSERT 1 = 'todo'.
ENDMETHOD.

so asking a parsed node which namespace it belongs to ends the program rather
than answering:

parse( |<n:root xmlns:n="urn:x"><n:item>A</n:item></n:root>| )->get_root_element( )->get_namespace_uri( ).
" ASSERTION_FAILED

The prefix itself is readable — get_namespace_prefix returns n — but a
prefix says nothing on its own: it is local to the document, and the uri is
what identifies the namespace. Anything that has to tell one namespace from
another (a SOAP envelope, an OData document, an abapGit file) needs the uri,
and today cannot ask for it.

The change

The declaration that binds a prefix is an ordinary attribute:
xmlns:<prefix>, or xmlns for a node without a prefix. It may stand on the
node itself or on any of its ancestors, and the nearest one wins — so the
lookup walks up the parents and returns the first binding it finds:

IF mv_namespace IS INITIAL.
  lv_name = 'xmlns'.
ELSE.
  CONCATENATE 'xmlns:' mv_namespace INTO lv_name.
ENDIF.

li_node = me.
WHILE li_node IS BOUND.
  li_map = li_node->get_attributes( ).
  IF li_map IS BOUND.
    li_attr = li_map->get_named_item( lv_name ).
    ...

An undeclared prefix has no uri: