NetHttpPersistent: ConnectionError on OpenTimeout

[olleolleolle]

Description

This PR adds a special-case from the net-http adapter to the net-http-persistent adapter.

open timeout was treated as a failure to connect, but our tests required it to be a timeout. Turns out, that was a call which was necessary before 4.0.0 of net-http-persistent.

There was a 4.0 released of net-http-timeout.

This change fixes the build by reraising as ConnectionError, just like in Faraday's net-http adapter.

Fixes #1152 by making the build pass.

Questions for reviewer

• The 4.0.0 release requires Ruby 2.5 (I think that it mentions the 2.4 going out of support as a reason to be able to let go of the exception handling it had in itself)
• This is a breaking change for this adapter

[olleolleolle]

@iMacTia Does this rhyme with how the other adapters handle open timeout?

[olleolleolle]

I took a look at the history, and learned that the net-http adapter treats it as a ConnectionError on purpose: 9ad0fcd

[olleolleolle]

Learning more: There was a recent release of net-http-persistent, which got in a new change. drbrain/net-http-persistent@2d23622

[iMacTia]

Thanks @olleolleolle for tackling this.
I had a closer look at the whole change and I suspect this change is being backwards-incompatible.
Tests will be passing, however updating Faraday while using net-http-persistent < 4.0 will probably break, as it will raise a different exception from before in case of timeout?
With this change, we basically stop testing for v3.0 and change the code to fit only v4.0

Feel free to point out if I'm misunderstanding.

[olleolleolle]

Yes, to make this work for < 3.x + 3.x + 4.x, we need a new conditional in the "rescue and re-raise" section which this PR changed (and its tests).

[iMacTia]

I THINK this is now fixed in #1226, although to keep backwards-compatibility I've kept Faraday::Timeout as the exception instead of Faraday::ConnectionFailed