Avoiding duplicate and allowing unset of codec parameter

Avoiding duplicate and allowing unset of codec parameter.

This allows unsetting Opus FEC or other codec parameter. This also avoids duplication.
That is true and I was worried too. So I tried it, and it worked.
I think SDP parser did it right.

But I do think removing ";" is better.

BTW, I am confused by the adding operation that has been there (line 1027 under this CL): what would happen if the parameter to add is to sit as the first? Is there an additional ";"?