Rename Optional-Parts

Re: Second comment in section 2.2.3 Optional Parts of HTTP Adaptation:

> (XXX: we should probably rename Optional to something like Auxiliary
> or FYI because it is awkward/wrong that optional parts become 
> mandatory/required).
I agree, it is awkward that optional parts become mandantory after negotiation.
"Auxiliary parts" sounds good to me.

Shall we rename the named parameter "Optional-Parts" to "Aux-Parts"?

Regards
Martin