message: grpc-json: fix unit test response headers
  In `GrpcJsonTranscoderFilterTest`, remove `:path` from the response
  headers.
diff: @@ -258,12 +258,10 @@ TEST_F(GrpcJsonTranscoderFilterTest, NoTranscoding) {
  EXPECT_EQ(Http::FilterTrailersStatus::Continue, filter_.decodeTrailers(request_trailers));
  Http::TestHeaderMapImpl response_headers{{"content-type", "application/grpc"},
  - {":status", "200"},
  - {":path", "/grpc.service/UnknownGrpcMethod"}};
  + {":status", "200"}};
  Http::TestHeaderMapImpl expected_response_headers{{"content-type", "application/grpc"},
  - {":status", "200"},
  - {":path", "/grpc.service/UnknownGrpcMethod"}};
  + {":status", "200"}};
  EXPECT_EQ(Http::FilterHeadersStatus::Continue, filter_.encodeHeaders(response_headers, false));
  EXPECT_EQ(expected_response_headers, response_headers);
  