Avoid checking defined?(@html_safe) on Ruby 3+

[jhawthorn]

We should not need to check defined? here because we are only interested in whether @html_safe is truthy or falsy.

In Ruby versions >= 3.0, there is no warning on access of undefined ivars. On previous versions, it can emit a warning.

cc @casperisfine @noahgibbs

$ cat benchmark_html_safe.rb
require 'rails'
require 'action_view'
require 'benchmark/ips'

SAFE_BUFFER = ActiveSupport::SafeBuffer.new

Benchmark.ips do |x|
  x.report("SAFE_BUFFER.html_safe?", "SAFE_BUFFER.html_safe?")
end

Before

$ be ruby benchmark_html_safe.rb
Warming up --------------------------------------
SAFE_BUFFER.html_safe?
                         1.300M i/100ms
Calculating -------------------------------------
SAFE_BUFFER.html_safe?
                         12.884M (± 0.8%) i/s -     65.005M in   5.045850s

After

$ be ruby benchmark_html_safe.rb
Warming up --------------------------------------
SAFE_BUFFER.html_safe?
                         2.456M i/100ms
Calculating -------------------------------------
SAFE_BUFFER.html_safe?
                         24.336M (± 1.0%) i/s -    122.821M in   5.047382s

[noahgibbs]

LGTM either way.

[jhawthorn]

With Jean's attr_reader suggestion.

$ be ruby benchmark_html_safe.rb
Warming up --------------------------------------
SAFE_BUFFER.html_safe?
                         2.989M i/100ms
Calculating -------------------------------------
SAFE_BUFFER.html_safe?
                         29.188M (± 1.3%) i/s -    146.468M in   5.018946s

[jhawthorn]

Actually realizing we need remove_method, not undef_method, because we want this to fall through to ::String's implementation of html_safe 😅

[byroot]

Right, I always have to check the doc when I need either of these. I can never remember which is which.