Fix get_buffer_include_files(bufnumber) for python

Currently executing command:

:echo g:neocomplete#sources#include#paths

prints: {'python3': ',', 'python': ''}

The problem is that both are obviously empty and only 'python' path is cleaned
(by removing extra ',' and duplicate paths). With this patch both python and
python3 include paths are set correctly and both cleaned.

This gives, for example on my system:

:echo g:neocomplete#sources#include#paths

{'python3':
'/usr/lib/python3.3/site-packages,/usr/lib/python34.zip,/usr/lib/python3.4,
/usr/lib/python3.4/plat-linux,/usr/lib/python3.4/lib-dynload,
/usr/lib/python3.4/site-packages',
'python':
'/usr/lib/python3.3/site-packages,/usr/lib/python34.zip,/usr/lib/python3.4,
/usr/lib/python3.4/plat-linux,/usr/lib/python3.4/lib-dynload,
/usr/lib/python3.4/site-packages'}

The both are same because in this case as 'python' is python 3 on my system.