Add Dream.with_client_stream

Primarily useful for the client, so it should be moved to dream-core in
the future. Will also be useful for stream-transforming middlewares.

src/dream.mli

@@ -733,6 +733,8 @@ val server_stream : 'a message -> stream
 val client_stream : 'a message -> stream
 (* TODO Document that this is for middlewares that are transforming a response
    stream or a WebSocket. *)
+val with_client_stream : stream -> 'a message -> 'a message
+(* TODO Normalize with with_stream, or add a separate with_server_stream. *)
 
 (* TODO Probably even close can be made optional. exn can be made optional. *)
 (* TODO Argument order? *)

src/pure/inmost.ml

@@ -298,6 +298,9 @@ let client_stream message =
 let server_stream message =
   message.server_stream
 
+let with_client_stream client_stream message =
+  update {message with client_stream}
+
 (* TODO Pending the dream.mli interface reorganization for the new stream
    API. *)
 let next =
@@ -499,8 +502,8 @@ let request
       upload = initial_multipart_state ();
     };
     headers;
-    client_stream = Stream.(stream (string body) no_writer);
-    server_stream = Stream.(stream no_reader no_writer);
+    client_stream = Stream.(stream no_reader no_writer);
+    server_stream = Stream.(stream (string body) no_writer);
     locals = Scope.empty;
     first = request;
     last = ref request;