fix: reject a ListBuckets bucket-region other than the gateway region

S3 never lists an empty result for another region: a valid region
other than the endpoint's own gets 400 InvalidArgument, "Requests with
bucket-region specified must be made to the corresponding regional
endpoint", with no ArgumentValue. The gateway is a single regional
endpoint, so it now does the same instead of clearing the listing.

Match the region case-insensitively like S3, and check an empty
bucket-region= too by looking for the query argument itself, since an
empty value is not a valid AWS region either.

s3api/controllers/bucket-list_test.go

@@ -119,25 +119,46 @@ func TestS3ApiController_ListBuckets(t *testing.T) {
 			name: "bucket region of another region",
 			input: testInput{
 				locals: defaultLocals,
-				beRes: s3response.ListAllMyBucketsResult{
-					Owner:   validRes.Owner,
-					Buckets: validRes.Buckets,
-					Prefix:  "te",
-					// a continuation token from the backend must not
-					// leak into the empty listing
-					ContinuationToken: "test",
-				},
+				beRes:  validRes,
 				queries: map[string]string{
 					"bucket-region": "us-west-2",
-					"prefix":        "te",
 				},
 			},
 			output: testOutput{
 				response: &Response{
-					Data: s3response.ListAllMyBucketsResult{
-						Owner:  validRes.Owner,
-						Prefix: "te",
-					},
+					MetaOpts: &MetaOptions{},
+				},
+				err: s3err.GetInvalidArgumentErr(s3err.InvalidArgBucketRegionEndpoint, ""),
+			},
+		},
+		{
+			name: "empty bucket region",
+			input: testInput{
+				locals: defaultLocals,
+				beRes:  validRes,
+				queries: map[string]string{
+					"bucket-region": "",
+				},
+			},
+			output: testOutput{
+				response: &Response{
+					MetaOpts: &MetaOptions{},
+				},
+				err: s3err.GetInvalidArgBucketRegion(""),
+			},
+		},
+		{
+			name: "bucket region of the gateway region in uppercase",
+			input: testInput{
+				locals: defaultLocals,
+				beRes:  validRes,
+				queries: map[string]string{
+					"bucket-region": "US-EAST-1",
+				},
+			},
+			output: testOutput{
+				response: &Response{
+					Data: validRes,
 				},
 			},
 		},