RichText model extensions are not supported

[JaimeSA]

When using dxa 2.2 (.net) and creating a custom RichText Model YourRichTextModelHere that extends RichText, DXA fails mapping any field using that extended model as the code expects always a RichText model, not a sub-class of it.

The full details of the issue can be found here:
https://tridion.stackexchange.com/questions/20327/does-dxa-support-extended-richtext-models-is-this-by-design/20328#20328