Lower case the NSEC next owner name when signing

Lower case the NSEC next owner name when signing

When building the NSEC rdata, lower case the next owner name before
storing it in the Next Domain Name Field.

Note that this is not required according to RFC 6840, Section 5.1:

  When canonicalizing DNS names (for both ordering and signing), DNS
  names in the RDATA section of NSEC resource records are not converted
  to lowercase.  DNS names in the RDATA section of RRSIG resource
  records are converted to lowercase.

  The guidance in the above paragraph differs from what has been
  published before but is consistent with current common practice.
  Item 3 of Section 6.2 of [RFC4034] says that names in both of these
  RR types should be converted to lowercase.  The earlier [RFC3755]
  says that they should not.

Since there is inconsistency in the documents over time, having
uppercase next owner names in the NSEC records may cause validation
failures if validators are not implementing RFC 6840.

Also, RFC 4034 section 6.2 is not about how NSEC record content is
created, but how RRset content is normalized in order to produce and
validate RRSIG records for a given RRset. Since the next owner name
of the NSEC record is about ordening, and the canonical DNS name order
requires that uppercase US-ASCII letters must be treated as if they
were lowercase US-ASCII letters, case is not meaningful for NSEC
next owner names, as it cannot be compressed on the wire, so we may
lowercase the next owner name in the NSEC rdata before signing, being
more kind to validators.