Update typeclass list for getresource

[themichaelyang]

Hi! I noticed that the documentation for getresource was missing a number of supported DNS resource types.

I also added a test to check every supported resource type.

Here's the irb snippet I wrote to get the typeclasses missing from the docs:

require "resolv"

documented = <<EOF
    # * Resolv::DNS::Resource::IN::A
    # * Resolv::DNS::Resource::IN::AAAA
    # * Resolv::DNS::Resource::IN::ANY
    # * Resolv::DNS::Resource::IN::CNAME
    # * Resolv::DNS::Resource::IN::HINFO
    # * Resolv::DNS::Resource::IN::MINFO
    # * Resolv::DNS::Resource::IN::MX
    # * Resolv::DNS::Resource::IN::NS
    # * Resolv::DNS::Resource::IN::PTR
    # * Resolv::DNS::Resource::IN::SOA
    # * Resolv::DNS::Resource::IN::TXT
    # * Resolv::DNS::Resource::IN::WKS
EOF
.split("\n").map { _1.split("::").last }

actual = Resolv::DNS::Resource::IN.constants.map(&:to_s)

p documented - actual # => []
p actual - documented # => ["CAA", "SRV", "ClassValue", "ServiceBinding", "LOC", "SVCB", "HTTPS"]

# `ClassValue` and `ServiceBinding` are constants but not valid resource types