Net::HTTP to a closed port raises SocketError where MRI raises Errno::ECONNREFUSED #772

Net::HTTP.get to a closed port raises the wrong exception class.

require "net/http"
begin
  Net::HTTP.get(URI("http://127.0.0.1:1/"))
rescue Exception => e
  puts "raised #{e.class}"
end

Errno::ECONNREFUSED is what a program rescues to distinguish "nothing is listening" from a name-resolution or protocol failure, and retry logic is routinely written on exactly that distinction, so the substitution is not cosmetic: rescue Errno::ECONNREFUSED does not fire, and rescue SocketError catches both cases where MRI separates them.

Found alongside #771 while measuring the embedding surface for #768. Unlike #771 this one is a plain conformance divergence with no hang involved.

Worth checking the neighbouring cases in the same pass rather than only this one: ECONNRESET, EHOSTUNREACH, ETIMEDOUT and a genuine DNS failure (which should be SocketError), since whatever maps the Go error to a Ruby class here is likely to collapse those too. A census of the mapping would be better than a single-case fix.

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