Fix: keep `python-shell-interpreter-args' working.

On windows, IPython only works with following configuration:
(setq  python-shell-interpreter "C:\\Python27\\python.exe"
   python-shell-interpreter-args  "-i
   C:\\Python27\\Scripts\\ipython-script.py"))

Without the `python-shell-interpreter-args', python-django-mode's shell
doesn't have the prompt "In [*]:" and its  subprocess doesn't work right. So I
keep the `python-shell-interpreter-args' here.