http: reduce network traffic for authorization

http: reduce network traffic for authorization

- Authorization: header never changes for Basic authentication scheme,
  so simply cache it. For Office 365 this reduces the network traffic
  considerably, because we only get one 401 response for the first
  request on a connection and not for every new request.

- don't throw away the security context unnecessarily. F.ex. a missing
  photo fetch returns 404, but that shouldn't invalidate the security
  context. Just throw it away for status codes 403, 407 and >= 500.