Addresses a bug in the file plugin where SOA queries to zone delegations are
inappropriately returned the SOA for the delegating zone, and not a downward
referral to the delegated zone.

Here is an example of what I believe the expected downward referral in response
to a SOA query for a delegated zone should be (note that no SOA record is
returned):

~~~
; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> @k.root-servers.net. miek.nl. SOA
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58381
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 3, ADDITIONAL: 7
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;miek.nl.                       IN      SOA

;; AUTHORITY SECTION:
nl.                     172800  IN      NS      ns1.dns.nl.
nl.                     172800  IN      NS      ns2.dns.nl.
nl.                     172800  IN      NS      ns3.dns.nl.

;; ADDITIONAL SECTION:
ns1.dns.nl.             172800  IN      A       194.0.28.53
ns2.dns.nl.             172800  IN      A       194.146.106.42
ns3.dns.nl.             172800  IN      A       194.0.25.24
ns1.dns.nl.             172800  IN      AAAA    2001:678:2c:0:194:0:28:53
ns2.dns.nl.             172800  IN      AAAA    2001:67c:1010:10::53
ns3.dns.nl.             172800  IN      AAAA    2001:678:20::24
~~~

See #3852 for the original fix.

Modified clouddns/route53 and removed the faulty tests there.
Signed-off-by: Miek Gieben <miek@miek.nl>