Added RFC2308 compliant $TTL directive behavior.
The directive is used to set the TTL of all resource records that do not explicitly set a TTL.
When a $TTL directive is not set, the original behavior of inheriting the previously set TTL is used.
When neither a $TTL directive or explicit RR TTL is set, the minimum TTL of the SOA record is used.
An additional test dns-zoneparse-dollar-ttl.t has be added to prove that the original behavior is still working.
The main test dns-zoneparse.t has been updated to reflect the new behavior as the test zone test-zone.db uses a $TTL directive.
Both of these tests prove the functionality to work in either case.
Merge branch 'master' of https://github.com/xdrr/dns-zoneparse-perl