Add dns_nameserver option for mx checking

[timkrins]

Adds an option for easily customising the DNS resolver that is used for checking A and MX records.
If no option is given, it will use all current defaults.

[micke]

Thanks! I just have some feedback!

[timkrins]

Review comments resolved

[micke]

@timkrins great! Seems like you have some failing tests still!

[timkrins]

Ensuring the timeout test pass is an interesting one - the DNS response was too quick in CI to hit the timeout, but did hit the timeout locally.

My new solution to fail the DNS resolution in time seems to be a bit 'hacky'...

As long as the DNS server doesn't respond in less than 2.2250738585072014e-308 seconds (Float::MIN), and resolv respects a float value, the test should now work (the timeout value is validated by resolv and must be positive).

The only other way to cause a timeout might be to start a socket on a thread that does not reply (this is how it is done in the resolv lib tests), but this is quite a deviation from the simpleness of the current tests.

[micke]

I've pushed 5.2.0 to rubygems. Thanks!