fromLegacyHref
Creates an Url from a legacy HREF.
For example, if it is a relative path such as /dir/my chapter.html, it will be converted to the valid relative URL dir/my%20chapter.html.
Only use this API when you are upgrading to Readium 3.x and migrating the HREFs stored in your database. See the 3.0 migration guide for more information.