disable utf-8 escaping in json.stringify by default & add utfEscape argument to json.stringify & update network.post behaviour for pre-serialized data

src/logic/EngineController.cpp

@@ -178,7 +178,7 @@ void EngineController::onMissingContent(const std::shared_ptr<ContentReport>& re
     if (engine.isHeadless()) {
         throw std::runtime_error(
             "missing content: " +
-            json::stringify(create_missing_content_report(report), true)
+            json::stringify(create_missing_content_report(report), true, "  ")
         );
     } else {
         engine.setScreen(std::make_shared<MenuScreen>(engine));

src/logic/scripting/lua/libs/libjson.cpp

@@ -5,7 +5,8 @@ static int l_json_stringify(lua::State* L) {
     auto value = lua::tovalue(L, 1);
 
     bool nice = lua::toboolean(L, 2);
-    auto string = json::stringify(value, nice, "  ");
+    bool escapeUTF = lua::toboolean(L, 3);
+    auto string = json::stringify(value, nice, "  ", escapeUTF);
     return lua::pushstring(L, string);
 }
 

src/logic/scripting/lua/libs/libnetwork.cpp

@@ -44,7 +44,12 @@ static int l_post(lua::State* L) {
     lua::pushvalue(L, 3);
     auto onResponse = lua::create_lambda_nothrow(L);
 
-    auto string = json::stringify(data, false);
+    std::string string;
+    if (data.isString()) {
+        string = data.asString();
+    } else {
+        string = json::stringify(data, false);
+    }
     engine->getNetwork().post(url, string, [onResponse](std::vector<char> bytes) {
         auto buffer = std::make_shared<util::Buffer<ubyte>>(
             reinterpret_cast<const ubyte*>(bytes.data()), bytes.size()