New probe server fixes: allow accept_nonblock to raise and IO.gets return nil

[stas]

A couple of suggested fixes for the new probe http server. Commits should be self-explanatory, still some more context below.

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Passing exceptions: false no longer raises IO::WaitReadable leaving the server in an unknown state.

By specifying a keyword argument exception to false, you can indicate that accept_nonblock should not raise an IO::WaitReadable exception, but return the symbol :wait_readable instead.

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An IO.gets returns either a string or a nil. A nil response from a socket was not handled, suggesting we just skip those.

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Appreciate taking a look as we're affected by these issues.

[bensheldon]

@stas thank you! This change makes sense. I guess we were using exception: false but not checking if the return value == :wait_readable. From reading the docs, it wasn't entirely clear if there were other classes of exceptions that could be raised. But I'm ok accepting this and we'll see what we learn next 👍🏻

[bensheldon]

@stas I just released this in https://github.com/bensheldon/good_job/releases/tag/v3.17.3