lua: kKeep type names to avoid incorrect require casing

Having a struct UUID, the file generated was Uuid.lua,
but generated code passing UUID all uppercase to require,
on case sensitive filesystems this doesn't work

src/bfbs_gen_lua.cpp

@@ -46,7 +46,7 @@ std::set<std::string> LuaKeywords() {
 }
 
 Namer::Config LuaDefaultConfig() {
-  return { /*types=*/Case::kUpperCamel,
+  return { /*types=*/Case::kKeep,
            /*constants=*/Case::kUnknown,
            /*methods=*/Case::kUpperCamel,
            /*functions=*/Case::kUpperCamel,