Fix NSEC record conversion
NSEC record needs special treatment as it is not composed from
a fixed set of DNS parts divided by space, but it contains
a multivalued DNS part "types" containing a list of RR types
it covers.
There was already a special method for parsing raw NSEC record
to DNS parts, but the other direction was missing. This patch
adds special NSEC convertor to fix this issue.
https://fedorahosted.org/freeipa/ticket/2307