stripe: support creating checkout session with all dicts and lists serialized

```py
kw = {...}
kw['metadata[key]'] = 'value'
connector.createSession(**kw)
```

but it would be more beautiful to be able to write it like this:

```py
kw = {...}
kw['metadata] = {'key': 'value'}
connector.createSession(**kw)
```

This would need change in the way the request data is serialised in
createSession.