Validating all prefixes from received server answers

I am not sure about some steps that I've done here. 

For instance, in situation where insist-mode is off, client includes two IA_PD options in SOLICIT and in ADVERTISE one of IA_PD has 0 valid prefixes, I delete that  IA_PD option. if not, IA_PD without prefixes would be included in REQUEST message, because I always delete IA_Prefix option when one is not valid. 

I used Forge to test that code. It would be nice if you would take a quick look at descriptions of each test. I hope that tests cover every case relative to validating prefixes and submitted code.
https://github.com/isc-projects/forge/tree/client_pd_testing/lettuce/features/dhcpv6/client/dibbler_only

I might have added too many comments in code.