Set default_aa on AsyncDnsServer to False by default

Set default_aa on AsyncDnsServer to False by default

In 6e684d44 I mistakenly set the default for `default_aa` for
`AsyncDnsServer()` to `True` and then explicitly set it to True in
cases where all the `ResponseHandlers` said
`yield DnsResponseSend(..., authoritative=True)` as if the default was
`False`.

Also the rest of `AsyncDnsServer` code (namely `_prepare_responses`)
reads like `default_aa` is `False` by default.

This accidentally changed the behavior of servers which don't set the
`default_aa` and where AA is not set from the zone data
(e.g. `dispatch/ans3`).