WIP: stripe: support creating checkout session with metadata

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

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

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

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