HttpClient: a route's x-method_name can overwrite the client's own attributes #23

Found by the adversarial review of #22, and also present before it.

HttpClient.register_method does setattr(self, func.__name__, ...) with a name taken from the spec (x-method_name). A spec, possibly fetched from a server, can therefore name a route openapi_spec, binary_loads, session, handle_request or ensure_login and replace that attribute of the client. For example, a route named openapi_spec makes api_pkg_files crash with 'function' object has no attribute 'get'.

Suggested fix: refuse, or prefix, route names that collide with HttpClient attributes or start with _. Better still, keep the routes in a separate namespace (e.g. client.routes), with attribute access kept as sugar for names that do not collide.