Allow setting write_timeout for connections on Ruby 2.6+

[bdewater]

See original Ruby issue: https://bugs.ruby-lang.org/issues/13396 and implementation: ruby/ruby@bd7c46a

In Ruby 2.6+ this defaults to 60s. I picked 30 seconds to make it in line with open_timeout. IMO the defaults are too generous but that's a discussion for another time :)

[CLAassistant]

All committers have signed the CLA.

[brandur-stripe]

Thanks @bdewater! This looks good to me. Do you think that in StripeConfiguration#write_timeout= we could error if someone tries to set it in Ruby < 2.6? It doesn't seem like a good idea to make it settable but have it do nothing in case people are not on 2.6 yet, and most users probably aren't.

IMO the defaults are too generous but that's a discussion for another time :)

Hah, +1, but yep, let's defer that conversation.

[bdewater]

Feedback addressed and CLA signed.

[brandur-stripe]

Thanks! This LGTM.

[brandur-stripe]

Released as 5.28.0.