title: flask-security crash on itsdangerous
body: <p>I'm pretty sure it is me and not a bug - but assitance in figuring it out would be appreciated</p>
  
  <p>I'm getting a crash in itsdangerous</p>
  
  <pre><code>    def derive_key(self):
          """This method is called to derive the key.  If you're unhappy with
              the default key derivation choices you can override them here.
              Keep in mind that the key derivation in itsdangerous is not intended
              to be used as a security method to make a complex key out of a short
              password.  Instead you should use large random secret keys.
              """
          salt = want_bytes(self.salt)
          if self.key_derivation == 'concat':
              return self.digest_method(salt + self.secret_key).digest()
          elif self.key_derivation == 'django-concat':
              print salt
              print self.secret_key
              return self.digest_method(salt + b'signer' +
  &gt;               self.secret_key).digest()
  
   TypeError: cannot concatenate 'str' and 'NoneType' objects
  </code></pre>
  
  <p>.env/lib/python2.7/site-packages/itsdangerous.py:336: TypeError
  and the relevant traceback part is:</p>
  
  <pre><code>flask_app/mgmt_admin.py:28: in login
      return jsonify(success=True, token=user.get_auth_token())
  .env/lib/python2.7/site-packages/flask_security/core.py:313: in get_auth_token
      return _security.remember_token_serializer.dumps(data)
  .env/lib/python2.7/site-packages/itsdangerous.py:568: in dumps
      rv = self.make_signer(salt).sign(payload)
  .env/lib/python2.7/site-packages/itsdangerous.py:414: in sign
      return value + sep + self.get_signature(value)
  .env/lib/python2.7/site-packages/itsdangerous.py:349: in get_signature
      key = self.derive_key()
  </code></pre>
  
  <p>I'm defining the app pretty straight forward:</p>
  
  <pre><code>app.config["SECRET_KEY"] = "testing_key"
  app.config["TESTING"] = True
  </code></pre>
  
  <p>from printing the values I see that:</p>
  
  <pre><code>salt: remember-salt
  self.secret_key: None
  </code></pre>