bucketExists: Return consistent NotFound error code

bucketExists() currently returns NoSuchBucket 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 +647,15 @@ export class Client {
       throw new TypeError('callback should be of type "function"')
     }
     var method = 'HEAD'
-    this.makeRequest({method, bucketName}, '', 200, '', false, cb)
+    var pathStyle = true
+    this.makeRequest({method, bucketName, pathStyle}, '', 200, 'us-east-1', false, (e) => {
+      if (e && e.code == 'MovedPermanently') {
+        // MovePermanently status code implicitly says the bucket exists
+        // in a specific region, therefore we return a success here.
+        cb(null)
+      }
+      cb(e)
+    })
   }
 
   // Remove a bucket.

src/test/functional/functional-tests.js

@@ -191,7 +191,7 @@ describe('functional tests', function() {
     step(`bucketExists(bucketName, cb)_bucketName:${bucketName}_`, done => client.bucketExists(bucketName, done))
     step(`bucketExists(bucketName, cb)_bucketName:${bucketName}random_`, done => {
       client.bucketExists(bucketName + 'random', (e) => {
-        if (e.code === 'NoSuchBucket') return done()
+        if (e.code === 'NotFound') return done()
         done(new Error())
       })
     })