missing key in saml request

There have been some oopses related to SAML requests. It looks like the saml2idp code could be more robust against missing keys in the request.

A sample OOPS traceback is included

  MultiValueDictKeyError: "Key 'SAMLRequest' not found in <QueryDict: {u'ec': [u'302'], u'startURL': [u'/a0TD0000006T0g8']}>"

      File "/srv/login.ubuntu.com/production/canonical-identity-provider-rev-1069/lib/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/srv/login.ubuntu.com/production/canonical-identity-provider-rev-1069/lib/django/views/decorators/csrf.py", line 77, in wrapped_view
    return view_func(*args, **kwargs)
  File "/srv/login.ubuntu.com/production/canonical-identity-provider-rev-1069/src/ubuntu_sso_saml/views.py", line 14, in saml_begin
    return login_begin(request)
  File "/srv/login.ubuntu.com/production/canonical-identity-provider-rev-1069/lib/django/views/decorators/csrf.py", line 77, in wrapped_view
    return view_func(*args, **kwargs)
  File "/srv/login.ubuntu.com/production/canonical-identity-provider-rev-1069/lib/saml2idp/views.py", line 49, in login_begin
    request.session['SAMLRequest'] = source['SAMLRequest']
  File "/srv/login.ubuntu.com/production/canonical-identity-provider-rev-1069/lib/django/utils/datastructures.py", line 295, in __getitem__
    raise MultiValueDictKeyError("Key %r not found in %r" % (key, self))