Fix UnicodeError when givenName or sn Shibboleth Header contains unicode chars

Shibboleth Header values like Nicol\xc3\xb2  raise UnicodeError evaluating fullname 
Yes...this is my stack trace when the user "Nicolò Vitale" try to login with Shibboleth without the code in this PR

```
Sep  2 18:55:06 web144 [service_variant=lms][edx.external_auth][env:sandbox] INFO [web144  25866] [views.py:527] - SHIB creds returned: {'REMOTE_USER': 'XXXX@mail.XX', 'displayName': '', 'Shib-Identity-Provider': 'MY_IDP_URL', 'sn': 'Vitale', 'mail': 'nicolo.vitale@mail.XXX, 'givenName': 'Nicol\xc3\xb2'}
Sep  2 18:55:06 web144 [service_variant=lms][root][env:sandbox] ERROR [web144  25866] [exceptions.py:9] - Uncaught exception from <class 'django.core.handlers.wsgi.WSGIHandler'>
Traceback (most recent call last):
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/edx/app/edxapp/edx-platform/common/djangoapps/external_auth/views.py", line 529, in shib_login
    fullname = shib['displayName'] if shib['displayName'] else u'%s %s' % (shib['givenName'], shib['sn'])
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 5: ordinal not in range(128)
```