Fix char encoding error when setting shipping_type in session

[jaywink]

On Finnish locale, translation for shipping type has non-ascii letters and thus saving order gave a DB error.

[stephenmcd]

Don't think that's the right fix for Python 3.

[jaywink]

Yes you are right hmm. Of course reading the docs for future.builtins.str method this shouldn't even be a problem. I did a full compare with my fork and cartridge master and couldn't find anything. I then put a print vars(request.session) just after the utils.set_shipping method sets the values, and the following where you can clearly see the problem:

{'_session_cache': {u'tax_total': 0, '_auth_user_id': 1L, u'tax_type': 'Vero', u'shipping_total': 0,
u'cart': 6L, '_auth_user_backend': 'mezzanine.core.auth_backends.MezzanineBackend', 
u'shipping_type': 'Toimitus kiinte\xe4\xe4n hintaan', u'order': {'card_name': u'',
'shipping_detail_last_name': u'Robinson', 'remember': True, 'billing_detail_email': 
u'mail@remoded.real.email', 'shipping_detail_first_name': u'Jason', u'additional_instructions': 
u'', 'card_type': u'', 'step': 1, 'billing_detail_last_name': u'Robinson', 'billing_detail_phone': 
u'+358451234567', 'shipping_detail_phone': u'', 'same_billing_shipping': False, 
'billing_detail_first_name': u'Jason'}}, '_session_key': 'lco10q5tv7mgrpabogi76yvrp4v2brsw', 
'modified': True, 'serializer': <class 'django.contrib.sessions.serializers.PickleSerializer'>, 
'accessed': True}

My fork has from __future__ import unicode_literals and from future.builtins import str in utils.py when the problem happens.. Python is 2.7.5, future at 0.9.0..

[stephenmcd]

I'm able to set shipping/tax types with non-ascii chars and complete an order:

http://cl.ly/image/2I1943192b1Y

Suggest there's an issue elsewhere, please reply if you work it out.