Fix async http adapter

[bruno-]

Problem: the server in Async::HTTP::WebMockEndpoint#accept_socket was
hanging forever after the rspec example finished.
This is normal as Async::HTTP::Server (or any server) is never supposed
to end. It just continues waiting for new requests which, in this case,
never come.

Solution: use a 'transient' async task when starting a server.
Transient tasks do not block an exiting async reactor.
So, in this case a transient server will not hang forever.

Fixes #858

@ioquatix we talked about making this a draft PR so we can continue talking about a proper webmock <-> async-http solution. Since this fix is minimal I propose going for a merge.
We can continue thinking about a better approach here socketry/async-http#35

[bruno-]

@ioquatix I just saw your latest comment #858 (comment)

Here is the "expanded" code from this PR:

Async(transient: true) do  # <= This task IS the server's task
  server = Async::HTTP::Server.new(WebMockApplication, self)
  server.accept(socket, socket.remote_address)
end

Is the ensure block with task&.stop necessary? If yes, we'd need to insert another task.async do... between Async and Async::HTTP::Server.new in the code above.

[ioquatix]

It looks okay to me - let's try it as implemented and check how it works.

[bblimke]

@bruno- @ioquatix thank you both for investigating this and for the fix.

[bblimke]

@bruno- @ioquatix actually the build is now failing: https://travis-ci.org/github/bblimke/webmock/jobs/725160832
ArgumentError: unknown keyword: transient

[bruno-]

I'm checking this.
My guess is webmock is using an older version of async or async-http gems that don't support that keyword.

[bruno-]

@bblimke it seems travis is using async 1.24.2 https://travis-ci.org/github/bblimke/webmock/jobs/725160832#L626
That version is from February this year, while transient feature was introduced in April.
async-http version >= 0.48.0 specified in webmock gemfile is fine. I don't think that needs to be changed.

Can we bump travis version? Maybe delete travis' cache or something?

[bblimke]

@bruno- I now see that it's only a problem on Ruby 2.3 and 2.4 which are likely not compatible with latest async version.

2.3 and 2.4 are EOL so I guess I can drop support for them anyway.

[ioquatix]

I have an unpublished policy for open source, I do not support EOL Rubies.

[bruno-]

@bblimke looking forward to next webmock gem release that contains these fixes.

[bblimke]

This has now been released as version 3.9.0