gRPC: RST_STREAM(NO_ERROR) handling after "trailer only" responses.

Similarly to the problem fixed in 2096b21fcd10 (ticket #1792),
when a "trailer only" gRPC response (that is, a response with the
END_STREAM flag in the HEADERS frame) was immediately followed by
RST_STREAM(NO_ERROR) in the data preread along with the response
header, RST_STREAM wasn't properly skipped and caused "upstream
rejected request with error 0" errors.
Observed with "unknown service" gRPC errors returned by grpc-go.

Fix is to set ctx->done if we are going to parse additional data,
so the RST_STREAM(NO_ERROR) is properly skipped.  Additionally, now
ngx_http_grpc_filter() will complain about frames sent for closed
stream if there are any.

src/http/modules/ngx_http_grpc_module.c

@@ -1969,6 +1969,7 @@ ngx_http_grpc_filter_init(void *data)
         }
 
         u->length = 0;
+        ctx->done = 1;
 
     } else {
         u->length = 1;