Fix the non-protobuf scenario

src/python/grpcio/grpc/_cython/_cygrpc/aio/common.pyx.pxi

@@ -42,6 +42,8 @@ cdef bytes serialize(object serializer, object message):
 
     Failure to serialize is a fatal error.
     """
+    if isinstance(message, str):
+        message = message.encode('utf-8')
     if serializer:
         return serializer(message)
     else:

src/python/grpcio_tests/tests_aio/benchmark/benchmark_servicer.py

@@ -51,5 +51,5 @@ class GenericBenchmarkServicer(
         return self._response
 
     async def StreamingCall(self, request_iterator, unused_context):
-        async for request in request_iterator:
+        async for _ in request_iterator:
             yield self._response