Add test for #38

This tests that a string with ä in it round-trips properly

pgx/src/pgx_value.ml

@@ -268,8 +268,8 @@ let to_point = Option.map to_point'
 
 let of_string t = Some t
 
-let to_string_exn = required (fun t -> t)
 let to_string t = t
+let to_string_exn = required to_string
 
 let unit = Some ""
 

pgx_test/src/pgx_test.ml

@@ -469,6 +469,32 @@ module Make_tests (IO : Pgx.IO) = struct
               | _ -> assert false))
           >>| List.iter (fun () -> ())
         )
+      ; "UTF-8 partial round-trip", (fun () ->
+          (* TODO: We should probably add more round-trip tests *)
+          let expect = "test-ä-test" in
+          with_conn (fun db ->
+            simple_query db "CREATE TEMPORARY TABLE this_test (id text)"
+            >>= fun _ ->
+            execute db ~params:[ Pgx.Value.of_string expect]
+              "INSERT INTO this_test (id) VALUES ($1)"
+            >>= fun _ ->
+            execute db "SELECT id FROM this_test"
+            >>| function
+            | [[ result ]] -> assert_equal (Some expect) (Pgx.Value.to_string result)
+            | _ -> assert false
+          )
+        )
+      ; "UTF-8 round-trip", (fun () ->
+          (* TODO: We should probably add more round-trip tests *)
+          let expect = "test-ä-test" in
+          with_conn (fun db ->
+            execute db ~params:[ Pgx.Value.of_string expect ]
+              "SELECT $1::VARCHAR"
+            >>| function
+            | [[ result ]] -> assert_equal (Some expect) (Pgx.Value.to_string result)
+            | _ -> assert false
+          )
+        )
       ] in
     make_tests "pgx_async" tests
     >>| run_test_tt_main ~exit