Fix for regression introduced in d0b7724f

While some trackers automatically handle decoding encoded URL attributes
others (primarily for performance reasons) stick very close to the spec.

When implementing the &ipv4= parameter I mistakenly added encoding for
symbols to IPv4 when BEP7 only specified that that should be the case
for IPv6. IPv4 is meant to be sent without encoding according to BEP7
and other client behaviour.