Passing JSON value other than an object in request body results in 500 Internal Server Error

[ppik]

When the client sends a POST request that consists of any other JSON value besides a JSON object, e.g, a string or an array, the endpoints framework returns with 500 Internal Server Error. Would expert a 400 Bad Request error for parsing ProtoRPC request.

To reproduce, e.g., use the echo example.
Example queries returning 500:

curl https://<YOUR-PROJECT-ID>.appspot.com/_ah/api/echo/v1/echo -H "Content-Type: application/json" -d '"hello world"'

curl https://<YOUR-PROJECT-ID>.appspot.com/_ah/api/echo/v1/echo -H "Content-Type: application/json" -d '["hello", "world"]'

Log message:

Traceback (most recent call last):
  File "/base/alloc/tmpfs/dynamic_runtimes/python27g/7679791231c143f9/python27/python27_lib/versions/1/google/appengine/runtime/wsgi.py", line 267, in Handle
    result = handler(dict(self._environ), self._StartResponse)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints_management/control/wsgi.py", line 166, in __call__
    return self.wsgi_backend(environ, start_response)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints_management/control/wsgi.py", line 278, in __call__
    return self._application(environ, start_response)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints_management/control/wsgi.py", line 723, in __call__
    return self._application(environ, start_response)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints_management/control/wsgi.py", line 415, in __call__
    result = b''.join(result)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints/endpoints_dispatcher.py", line 147, in __call__
    yield self.dispatch(request, start_response)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints/endpoints_dispatcher.py", line 170, in dispatch
    return self.call_backend(request, start_response)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints/endpoints_dispatcher.py", line 337, in call_backend
    orig_request, params, method_config)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints/endpoints_dispatcher.py", line 510, in transform_request
    request = self.transform_rest_request(orig_request, params, method_params)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints/endpoints_dispatcher.py", line 641, in transform_rest_request
    self._update_from_body(body_json, request.body_json)
  File "/base/data/home/apps/s~peeter-sandbox/20190418t084040.417569099234297404/lib/endpoints/endpoints_dispatcher.py", line 552, in _update_from_body
    for key, value in source.iteritems():
AttributeError: 'unicode' object has no attribute 'iteritems'"

For me it seems, that in the endpoints_dispatcher.py, _update_from_body functions just assumes that the given request.body_json is a dict without providing a graceful fallback.