Default to system nameservers if a Resolver is initialized without any.

[seandilda]

This change allows Resolver to be created without specifying nameservers. When no nameservers are specified, the system resolver config is used.

Types of Changes

• Quality of Life Improvement

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

Thanks, this is a reasonable change and a nice improvement.

[ioquatix]

I have been trying out this change, and I found the (my) implementation lacking.

Specifically, just using the system nameservers is insufficient, as there are other options like search domains, and so on, that are required. I've added Async::DNS::System.resolver which is also exposed by Async::DNS::Resolver.default.