fix(s3): use bucket's region in urlForObject()

This commit updates urlForObject() to use the bucket's region
instead of the stack's region when constructing the returned
URL.

Refs: aws#14149

packages/@aws-cdk/aws-s3/lib/bucket.ts

@@ -530,7 +530,7 @@ abstract class BucketBase extends Resource implements IBucket {
    */
   public urlForObject(key?: string): string {
     const stack = Stack.of(this);
-    const prefix = `https://s3.${stack.region}.${stack.urlSuffix}/`;
+    const prefix = `https://s3.${this.env.region}.${stack.urlSuffix}/`;
     if (typeof key !== 'string') {
       return this.urlJoin(prefix, this.bucketName);
     }

packages/@aws-cdk/aws-s3/test/bucket.test.ts

@@ -1599,10 +1599,15 @@ describe('bucket', () => {
   test('urlForObject returns a token with the S3 URL of the token', () => {
     const stack = new cdk.Stack();
     const bucket = new s3.Bucket(stack, 'MyBucket');
+    const bucketWithRegion = s3.Bucket.fromBucketAttributes(stack, 'RegionalBucket', {
+      bucketArn: 'arn:aws:s3:::explicit-region-bucket',
+      region: 'us-west-2',
+    });
 
     new cdk.CfnOutput(stack, 'BucketURL', { value: bucket.urlForObject() });
     new cdk.CfnOutput(stack, 'MyFileURL', { value: bucket.urlForObject('my/file.txt') });
     new cdk.CfnOutput(stack, 'YourFileURL', { value: bucket.urlForObject('/your/file.txt') }); // "/" is optional
+    new cdk.CfnOutput(stack, 'RegionBucketURL', { value: bucketWithRegion.urlForObject() });
 
     expect(stack).toMatchTemplate({
       'Resources': {
@@ -1678,6 +1683,20 @@ describe('bucket', () => {
             ],
           },
         },
+        'RegionBucketURL': {
+          'Value': {
+            'Fn::Join': [
+              '',
+              [
+                'https://s3.us-west-2.',
+                {
+                  'Ref': 'AWS::URLSuffix',
+                },
+                '/explicit-region-bucket',
+              ],
+            ],
+          },
+        },
       },
     });
 
@@ -2453,4 +2472,4 @@ describe('bucket', () => {
       autoDeleteObjects: true,
     })).toThrow(/Cannot use \'autoDeleteObjects\' property on a bucket without setting removal policy to \'DESTROY\'/);
   });
-});
+});