Improve TTL detection when reading master zone files.

Improve TTL detection when reading master zone files.

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.