httplib: make http serving with auth generate INFO messages on auth fail
httplib: make http serving with auth generate INFO messages on auth fail

2018/12/13 12:13:44 INFO  : /: 127.0.0.1:39696: Basic auth challenge sent
2018/12/13 12:13:54 INFO  : /: 127.0.0.1:40050: Unauthorized request from ncw

Fixes #2834

olihey/rclone,B4dM4n/rclone,ncw/rclone,olihey/rclone,ncw/rclone,olihey/rclone,olihey/rclone,B4dM4n/rclone,ncw/rclone,B4dM4n/rclone,B4dM4n/rclone,ncw/rclone