Otherwise `null` is returned which leads to a ton of similar error messages if the log condition setting is enabled.:
```
{"reqId":"3wVBarnDfOV3qkI1bVCU","remoteAddr":"89.98.78.41","app":"PHP","message":"hash_equals(): Expected user_string to be a string, null given at \/var\/www\/html\/lib\/private\/Log.php#275","level":3,"time":"2016-10-07T14:22:15+00:00","method":"PROPFIND","url":"\/remote.php\/webdav\/","user":"icewind"}
```
- Updated the config sample to point to log_type='file'
- Renamed the Class for logfile logging to File in namespace 'OC\Log\'.
Changed the occurrences of 'OC\Log\Owncloud' to 'OC\Log\File'.
- Renamed the Class for log:file command to File in namespace 'OC\Core\Command\Log\File'.
Changed registration of the command to use 'OC\Core\Command\Log\File'.
- Changed default Syslog tag to Nextcloud
- Retained backwards compatibility for configs with 'logtype' => 'owncloud'
- Adjusted tests for the new file log.
Closes#490.
