[Reviewer: Mike] Don't mangle PCFA headers that are IPv6 addresses

The bulk of this pull request is UTs that repro the issue - the rest is just tweaking PCFA parsing so that we use the paramchar spec for pvalues, not the token spec. (This is the difference between pjsip_parse_uri_param_imp and pjsip_parse_param_imp.)

Fixes https://github.com/Metaswitch/sprout/issues/611.