Fix some posix spawn warnings

[eileencodes]

Hello! 👋

We're seeing the first warning in our app and would like to fix this so that we can continue upgrading Ruby versions. If you don't want the other changes I'm happy to rebase them out since the first Object warning is the only one we actually need to be warning free in our application

This PR fixes 3 warnings coming from Ruby.

• Fixes /lib/posix/spawn.rb:532: warning: deprecated Object#=~ is called on Array; it always returns nil deprecation that appears on Ruby 2.7. This deprecation is also present in application code and not just gem code/test runs
• Fixes /test/test_spawn.rb:325: warning: ambiguous first argument; put parentheses or a space even after/' operator` which appears when tests are run.
• Fixes warning: loading in progress, circular require considered harmful loading error present in Ruby 2.6 and 2.7.

[tmm1]

Thanks!

[thepwagner]

@tmm1 would you be able to cut this into a release as well? (0.3.15 ?)

Also FYI this repo is missing the tag for v0.3.14 - https://github.com/rtomayko/posix-spawn/tags