How do I define a new entity for the HtmlUnit XML parser?

Question

I'm running into an issue with the HtmlUnit parser where I'm trying to grab some XML from a website (using the website's API) do a quick parse of the resulting XML and then save the XML to a file (all within the rights of the API). (sample content)

Unfortunately the website returns an entity ¿ in some of the requested pages, and while this is a valid HTML entity HtmlUnit is throwing an exception during the parse with message:

The entity "iquest" was referenced, but not declared.

How do I define iquest as a valid entity?