Brief summary of issue / Description of requested feature:

Json maps should load room symbols as well

Steps to reproduce the issue / Reasons for adding feature:

1. Have a map with a room with a symbol set to any random string
   
2. lua saveJsonMap(getMudletHomeDir() .. "/random_filename.json")
3. lua loadJsonMap(getMudletHomeDir() .. "/random_filename.json")
4. The room has no symbol anymore
   

Error output / Expected result of feature

4. The room is not changed

Extra information, such as Mudlet version, operating system and ideas for how to solve / implement:

Mudlet 4.12.0

The JSON still has the symbol, so saving works as expected: filename.json.txt

            "rooms": [
                {
                    "coordinates": [
                        0,
                        0,
                        0
                    ],
                    "environment": -1,
                    "exits": [
                    ],
                    "hash": "a054efe21738ea6ba27bd3689f013490",
                    "id": 1,
                    "symbol": {
                        "text": "M" -- there it is!
                    }
                }
            ]