bucketExists: return consistent NotFound error code

bucketExists() returns NoSucketBucket or NotFound error code depending
if region is specified in Minio Client initialization. This PR always
specifies us-east-1 region in HEAD bucket call to avoid the internal
code fetching for bucket location and returning NoSuchBucket error code.

src/main/minio.js

@@ -647,7 +647,7 @@ export class Client {
       throw new TypeError('callback should be of type "function"')
     }
     var method = 'HEAD'
-    this.makeRequest({method, bucketName}, '', 200, '', false, cb)
+    this.makeRequest({method, bucketName}, '', 200, 'us-east-1', false, cb)
   }
 
   // Remove a bucket.