Encoding error in session creation form

[wolfskaempf]

https://sentry.io/tom-wolfskampf/ga-statistics/issues/255986806/

UnicodeEncodeError: 'ascii' codec can't encode character u'\xe1' in position 60: ordinal not in range(128)
(15 additional frame(s) were not displayed)
...
  File "statistics/views/public_views.py", line 269, in create_session
    session.save()

UnicodeEncodeError: 'ascii' codec can't encode character u'\xe1' in position 60: ordinal not in range(128)