I would make responsibility separation.  I think `HTTPService __init__` should take care about all stuff which is 1) service specific, 2) common to all API calls.   So IMO `__init__()` responsibilities are:

1) accept service config and save it.
2) populate common data (and we should ensure it's at least semantically read-only)  for all API calls based on config.  In our case they are 'headers' and 'auth'.   So, I would move the part of current `update` method here.