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