HTMLText AbsoluteLink parse placeholders

Fixes #1910

Left AbsoluteLinks method in Text and overrode it in HTMLText so that original tests continue to pass, removing the AbsoluteLinks method in Text would have resulted in many more changes being required and could have resulted in unexpected behaviour for users.

The AbsoluteLinks method in HTMLText now parses shortcodes before passing the value to HTTP::absoluteLinks.

Added to the RSSFeedTest to ensure that things continue working as expected.