Add #flush and #sync methods to Puma::NullIO

[olivierbellone]

Description

Add #flush and #sync methods to Puma::NullIO.

This fixes an issue if you're using Events.null, e.g.:

i = Puma::Launcher.new(Puma::Configuration.new, {events: Puma::Events.null})
i.run

Without this patch, this would fail with:

NoMethodError: undefined method `sync' for #<Puma::NullIO:0x00007fb0cbb7cec8>
Did you mean?  sync=
from /Users/ob/workspace/ruby/puma/lib/puma/events.rb:67:in `log'

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[olivierbellone]

The "windows-2019 2.7 no SSL" CI failure looks unrelated, AFAICT.

[olivierbellone]

@nateberkopec Thanks for the quick review and merge! Do you think it would be useful to add more methods to NullIO to better mimic IO / StringIO, even if those methods are not used by Puma itself? If so I could open a followup PR.

[dentarg]

@olivierbellone I think so! I did #2520 after I found out NullIO didn't have #string, which I discovered while testing rack stuff.

[nateberkopec]

Seems like we'll need it eventually so that's fine by me @olivierbellone