Push custom certificate when registering WebHook
Push custom certificate when registering WebHook

There were two bugs here. Firstly we weren't passing the FieldFile
object to the Python Telegram API but None. Secondly, the Python
Telegram API most recent upgrade handles this type of file, whereas
they only handled BufferedReader before, see
python-telegram-bot/python-telegram-bot#119.

jlmadurga/django-telegram-bot,jlmadurga/django-telegram-bot