User-enabled target does not retain selection after editing

Issue Summary

When a user sets the checkbox for "Open URL in a New Window" this selection does not appear on subsequent edits. Instead, the checkbox always defaults to "checked" creating a scenario where every edit reverts the selection (assuming the editor did NOT intend for the link to open in a new window - otherwise, the issue is not evident)

The issue is that the $attributes of the element are not properly unserialized when displaying the form.

Patch forthcoming