title: python oauthlib: in escape ValueError "Only unicode objects are escapable"
body: <p>I'm using <code>python-social-auth</code> to login with social networks from my Django application. On my local machine everything works fine, but when I deploy to a server I get the following error: </p>
  
  <pre><code>oauthlib.oauth1.rfc5849.utils in escape
  ValueError: Only unicode objects are escapable. Got None of type &lt;type 'NoneType'&gt;.
  </code></pre>
  
  <p>Stacktrace:</p>
  
  <pre><code>File "django/core/handlers/base.py", line 112, in get_response
      response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "social/apps/django_app/utils.py", line 45, in wrapper
      return func(request, backend, *args, **kwargs)
  File "social/apps/django_app/views.py", line 12, in auth
      return do_auth(request.social_strategy, redirect_name=REDIRECT_FIELD_NAME)
  File "social/actions.py", line 25, in do_auth
      return strategy.start()
  File "social/strategies/base.py", line 66, in start
      return self.redirect(self.backend.auth_url())
  File "social/backends/oauth.py", line 99, in auth_url
      token = self.set_unauthorized_token()
  File "social/backends/oauth.py", line 158, in set_unauthorized_token
      token = self.unauthorized_token()
  File "social/backends/oauth.py", line 177, in unauthorized_token
      method=self.REQUEST_TOKEN_METHOD)
  File "social/backends/base.py", line 202, in request
      response = request(method, url, *args, **kwargs)
  File "requests/api.py", line 44, in request
      return session.request(method=method, url=url, **kwargs)
  File "requests/sessions.py", line 349, in request
      prep = self.prepare_request(req)
  File "requests/sessions.py", line 287, in prepare_request
      hooks=merge_hooks(request.hooks, self.hooks),
  File "requests/models.py", line 291, in prepare
      self.prepare_auth(auth, url)
  File "requests/models.py", line 470, in prepare_auth
      r = auth(self)
  File "requests_oauthlib/oauth1_auth.py", line 87, in __call__
      unicode(r.url), unicode(r.method), None, r.headers)
  File "oauthlib/oauth1/rfc5849/__init__.py", line 293, in sign
      request.oauth_params.append(('oauth_signature', self.get_oauth_signature(request)))
  File "oauthlib/oauth1/rfc5849/__init__.py", line 128, in get_oauth_signature
      uri, headers, body = self._render(request)
  File "oauthlib/oauth1/rfc5849/__init__.py", line 199, in _render
      headers = parameters.prepare_headers(request.oauth_params, request.headers, realm=realm)
  File "oauthlib/oauth1/rfc5849/utils.py", line 31, in wrapper
      return target(params, *args, **kwargs)
  File "oauthlib/oauth1/rfc5849/parameters.py", line 57, in prepare_headers
      escaped_value = utils.escape(value)
  File "oauthlib/oauth1/rfc5849/utilsy", line 56, in escape
      'Got %s of type %s.' % (u, type(u)))
  </code></pre>
  
  <p>Some packages from <code>requirements.txt</code> file:</p>
  
  <pre><code>Django==1.6.6
  google-api-python-client==1.1
  oauth2==1.5.211
  oauthlib==0.6.3
  python-openid==2.2.5
  python-social-auth==0.1.26
  requests==2.4.0
  requests-oauthlib==0.4.1
  six==1.7.3
  </code></pre>