docs: def payload_body(req):
      """
      A generator that will include the sha256 signature of the request's body
      in the JWT payload.  This is only done if the request could have a body:
      if the method is POST or PUT.
  
      >>> auth = JWTAuth('secret')
      >>> auth.add_field('body', payload_body)
      """
      to_hash = req.body if type(req.body) is bytes else req.body.encode('utf-8')
  
      if req.method in ('POST', 'PUT'):
          return {
                  'hash': hashlib.sha256(to_hash).hexdigest(),
                  'alg': 'sha256',
                  }