Now that we have 3 implementations of RTP header extensions, I realized that the caps code was either complex in one, and that the other didn't support the presence of the 3 part version of the caps. So I concluded that this should be common code that I moved into the base class. It also makes the subclasses a little simpler. The potential downside is that we now fix the format of the caps in the base class, but I this matches the RFC anyway.
I'm marking this as a Blocker as it's changing an API, so we need to decide if we like it before releasing 1.20.