[#2165] Only one log message now on check_access.

See: https://github.com/ckan/ckan/issues/2165

With this patch the logging changes from:
```
2014-12-29 10:41:28,743 DEBUG [ckan.logic] check access - user None, action get_site_user
2014-12-29 10:41:28,743 DEBUG [ckan.logic] Access OK.
```
to:
```
2014-12-29 11:05:06,988 DEBUG [ckan.logic] check access OK - get_site_user user=None
```