firebaseFunctions: calls to httpsCallable functions crash if payload is an object

[msn444]

On iOS, since version 11 of the plugin, calls to httpsCallable functions crash (with no error reporting) if the function is called with an object as the payload. Calls with string or null as payload work fine. Not an issue on Android.

[msaravitz]

x2, I've been stringifying the payload and parsing on the function.