JSON and stringified JSON bodies are treated the same offline but not online

[gurushida]

I found a problem similar to the one described in #702

If I have a lambda that expects JSON, I get a different behaviour online when I send the JSON like {"label":"foo"} vs when I send the stringified JSON "{\"label\":\"foo\"}".
However, I get the same behaviour offline where my lambda always receives an APIGatewayProxyEvent object where the body is a string containing
{"label":"foo"} regardless of what is sent.