[lwt] flush body writer before closing

On large requests, the writer sometimes closes before all data has been
written, which results in a truncated request (and usually an unexpected
EOF from the server).

Author: Firobe

lib/grpc-lwt/client.ml

@@ -84,6 +84,10 @@ module Rpc = struct
   let unary ~f enc write_body read_body =
     let payload = Grpc.Message.make enc in
     H2.Body.Writer.write_string write_body payload;
+    let wait_until_flushed, flushed = Lwt.wait () in
+    let callback _ = Lwt.wakeup flushed () in
+    H2.Body.Writer.flush write_body callback;
+    let* () = wait_until_flushed in
     H2.Body.Writer.close write_body;
     let* read_body = read_body in
     let request_buffer = Grpc.Buffer.v () in