Update: Created MutableHtmlData test to provide confidence in toString
methods which are used to display data to the user, also confidence in
equality methods for testing purposes.
Update: To pass required tests for MutableHtmlDataTest class, additional
functionality has been added to MutableHtmlData; more robust equality
case, more interesting toString and new hashcode. Furthermore the
hasOpeningTag and hasClosingTag have been added to the HtmlData
interface to allow for a more robust equals method.