Fix RPC calls with a form body in live mode

[hansallis]

Quick Summary:
RPC requests with an application/x-www-form-urlencoded body are transformed into JSON. In live mode however, strings were escaped as JSON and then surrounded (again) by quotes. This led to an invalid JSON string being sent to the browser window running the backend, causing a SyntaxError there and a timeout on the request.

SSCCE

1. lamdera init
2. Add the example RPC endpoint from the docs
3. lamdera live
4. POST a form:

curl -X POST http://localhost:8000/_r/hello-world \
   -H "Content-Type: application/x-www-form-urlencoded" \
   -d "param1=value1&param2=value2&param2=value3&test=4"

5. Observe the browser window running the backend:

SyntaxError: Expected ',' or '}' after property value in JSON at position 12 (line 1 column 13)
    at JSON.parse (<anonymous>)
    at M (...:68885:5691)
    at WebSocket.onmessage (...:68885:3228)

{
    "t": "q",
    "s": "9a2e567f38d61cf28c78aaa726c5e3b67e4bc828",
    "e": "hello-world",
    "r": "8a6204ff-dc23-43be-9ea3-b6a5b0a7f915",
    "h": {
        "content-length": "48",
        "content-type": "application/x-www-form-urlencoded",
        "accept": "*/*",
        "user-agent": "curl/8.7.1",
        "host": "localhost:8000"
    },
    "j": "{\"param1\":\"\"value1\"\",\"param2\":[\"\"value2\"\",\"\"value3\"\"],\"test\":\"\"4\"\"}"
}

[hansallis]

I've added a test to demonstrate, not sure if you want to keep that, but it was a fun Claude/Haskell co-op.