[BUG] Noweb reference eval syntax does not work

[fm4d <m@fm4d.net>]

The noweb reference syntax that should insert results of
evaluation of src block itself instead of src block as
described in `org-babel-expand-noweb-reference` does not
seems to work.


Code for replication:

* Assign 

First we assign abc:

#+begin_src python :noweb-ref assign_abc
abc = "abc" + "def"
#+end_src

* Use

Then we use it in a function:

#+begin_src python :noweb tangle :tangle noweb-test.py
def x():
  <<assign_abc>>
  return abc

print(x())
#+end_src


<<assign_abc>> does work, <assign_abc()> fails with this error:

(error "Reference ‘assign_abc’ not found in this buffer")
  signal(error ("Reference ‘assign_abc’ not found in this buffer"))
  error("Reference `%s' not found in this buffer" "assign_abc")
  org-babel-ref-resolve("assign_abc()")
  ...


I am not sure if this is a bug or I am doing something wrong,
the documentation on this feature is not very comprehensive.