ZrtpResponder no longer turns unrecognized agreement specs into DH3k

Also, the not-guaranteed-to-be-UTF8 data is no longer parsed as UTF8

Old behaviour: default to DH3k unless agreement spec was EC25, do decoder-dependent things on non-UTF8 data
New behaviour: accept EC25 or DH3k, else throw

Potential improvements: throw negotiation failed / bad packet (requires modifying several method signatures)