Rebuild doc_root in the build_tree

Because if attirbutes are updated/defined after __init__ the doc_root
didn't have that changes

Author: ecarreras

libcomxml/core/__init__.py

@@ -228,6 +228,7 @@ def build_tree(self):
         """
         if self.built:
             return
+        self.doc_root = self.root.element()
         for key in self.sorted_fields():
             if not key in self._fields:
                 continue