Add an HTTP header with the status code

[shs96c]

Currently, the only way for an intermediary node to determine whether it needs to clean state (eg. if an upstream session has disappeared unexpectedly) is by decoding the response. Providing more information could prevent this. Notably, encoding the status of the response as an HTTP header in responses would be useful.

I'd suggest X-WebDriver and the value being the value of the error field of an errored response. A value of success would also be added, to allow intermediary nodes to differentiate between Level 1 and Level 2 compliant upstream ends.