fix #1838 (#1839)

tools/goctl/api/dartgen/vars.go

@@ -58,7 +58,7 @@ Future _apiRequest(String method, String path, dynamic data,
       r = await client.getUrl(Uri.parse('https://' + serverHost + path));
     }
 
-    r.headers.set('Content-Type', 'application/json');
+    r.headers.set('Content-Type', 'application/json; charset=utf-8');
     if (tokens != null) {
       r.headers.set('Authorization', tokens.accessToken);
     }

tools/goctl/api/ktgen/apibase.tpl

@@ -25,7 +25,7 @@ suspend fun apiRequest(
         requestMethod = method
         doInput = true
         if (method == "POST" || method == "PUT" || method == "PATCH") {
-            setRequestProperty("Content-Type", "application/json")
+            setRequestProperty("Content-Type", "application/json; charset=utf-8")
             doOutput = true
             val data = when (body) {
                 is String -> {