autoDetectUrls needed for sizeThatFitsHTMLString as well

@mattdonnelly @yinsee 

this is a follow up to https://github.com/mattdonnelly/MDHTMLLabel/commit/054eb3d6244ddafa46d8e40a0f269b031969fd15

you'll want this as well, so that if you decide to disable autoDetectUrls, you can also disable it when calling sizeThatFitsHTMLString.

This is needed for correctness, but also performance, if you use MDHTMLLabel in a table view cell, you probably don't want to invoke the NSDataDetector as it can be slow.