This introduces the same behaviour that BIND has when it encounters implicit TTL
values, namely:
* The $TTL directive is preferred to set the default TTL [RFC2308].
* If no $TTL directive is seen prior to the SOA RR, the default TTL is set
to the minimum TTL field of the SOA RR [RFC1033, RFC1035, RFC1912].
* If neither $TTL nor an SOA are present, use the last seen explicit TTL on an
RR or raise a SyntaxError if no explicit TTLs have been seen.
Previously, when neither $TTL nor SOA were present, the TTL for an RR without
an explicit TTL would be set to 0 which is known to be a somewhat dangerous
value and shouldn't be encouraged as a default.
One test seems to rely on this default TTL=0 behaviour whilst testing something
seemingly unrelated (RR starting with whitespace), so that test has been
updated to match this new expectation.
Fixes #254.