Harshavardhana 964e354d06 Fix liveness check for NAS gateway (#7142) ...

Current master throws '503' unavailable for liveness check
```
~ curl -v http://localhost:9000/minio/health/live
> GET /minio/health/live HTTP/1.1
...
...
< HTTP/1.1 503 Service Unavailable
```

With this fix liveness check returns error appropriately
```
~ curl -v http://localhost:9000/minio/health/live
> GET /minio/health/live HTTP/1.1
...
...
< HTTP/1.1 200 OK
```

2019-01-24 19:14:05 +05:30

..

gateway-nas.go

Fix liveness check for NAS gateway (#7142)

2019-01-24 19:14:05 +05:30