fix: regression when listing buckets use the configured or default region

[prakashsvmx]

when listing buckets use the configured or default region

We fetch the region automatically only for buckets. so for listBuckets, either a region is required or use the default.

The latest version/master listBuckets does nor return any result . setting a default region would work as part of the client config.

var mc = new Minio.Client({
    endPoint: 'localhost',
    accessKey: 'minio',
    secretKey: 'minio123',
    useSSL: false,
    //partSize: 1024 * 1024 * 5,
    port: 22000,
    region:"us-east-1"
  })