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.

tests/test_rest_list_buckets.sh

@@ -285,12 +285,16 @@ export RUN_USERS=true
   run setup_bucket "$bucket_name"
   assert_success
 
-  local params=()
   if [ "$DIRECT" == "true" ]; then
-    params+=("-url" "https://s3.$test_region.amazonaws.com" "-awsRegion" "$test_region")
+    # the regional endpoint of that region lists only its own buckets
+    run list_buckets_bucket_not_in_list "$bucket_name" "$test_region" "-url" "https://s3.$test_region.amazonaws.com" "-awsRegion" "$test_region"
+    assert_success
+    return
   fi
 
-  run list_buckets_bucket_not_in_list "$bucket_name" "$test_region" "${params[@]}"
+  # the gateway is a single regional endpoint, so any other region is rejected
+  run send_rest_go_command_expect_error_with_specific_arg_name_value "400" "InvalidArgument" "Requests with bucket-region specified must be made to the corresponding regional endpoint" \
+   "ArgumentName" "bucket-region" "-query" "bucket-region=$test_region"
   assert_success
 }