Encoding::UndefinedConversionError when msf/sanity is required and sets encodings to binary and something explicitly sets the external encoding to utif-8

i18n-fail.log

Encoding.default_external: UTF-8
Encoding.default_internal: 

Encoding.default_external: ASCII-8BIT
Encoding.default_internal: ASCII-8BIT

Setting no encoding on file
f.external_encoding: ASCII-8BIT
f.internal_encoding: 
Contents: امثلة

Only setting external encoding on file
f.external_encoding: UTF-8
f.internal_encoding: ASCII-8BIT
U+0627 from UTF-8 to ASCII-8BIT (Encoding::UndefinedConversionError)

Setting external and internal encoding on file
i18n-fail.rb:21: warning: Ignoring internal encoding utf-8: it is identical to external encoding utf-8
f.external_encoding: UTF-8
f.internal_encoding: 
Contents: امثلة

Encoding.default_external: UTF-8
Encoding.default_internal: UTF-8

Only setting external encoding on file
f.external_encoding: UTF-8
f.internal_encoding: 
Contents: امثلة

i18n-fail.rb

# -*- encoding: utf-8 -*-

#
# CONSTANTS
#

PATH = 'arabic.txt'

#
# Methods
#

def read_arabic(encoding=nil)
  mode = "r"

  if encoding
    mode = "#{mode}:#{encoding}"
  end

  begin
    contents = File.open(PATH, mode) do |f|
      puts "f.external_encoding: #{f.external_encoding}"
      puts "f.internal_encoding: #{f.internal_encoding}"
      f.read
    end
  rescue Encoding::UndefinedConversionError => error
    puts "#{error} (#{error.class})"
  else
    puts "Contents: #{contents}"
  end
end

#
# Script
#

puts "Encoding.default_external: #{Encoding.default_external}"
puts "Encoding.default_internal: #{Encoding.default_internal}"

File.open(PATH, 'w') do |f|
  # 'examples' in Arabic
  # @see https://github.com/cucumber/gherkin/blob/ae88a979d0f29c25f6c0c25198d770a45d96d2ba/lib/gherkin/i18n.json#L37
  f.puts('امثلة')
end

# @see https://github.com/rapid7/metasploit-framework/blob/3a3d038904feb9729281af43e5ef97d2530d59aa/lib/msf/sanity.rb#L44
Encoding.default_external = Encoding.default_internal = "binary"

puts
puts "Encoding.default_external: #{Encoding.default_external}"
puts "Encoding.default_internal: #{Encoding.default_internal}"

puts
puts 'Setting no encoding on file'
read_arabic

puts
puts 'Only setting external encoding on file'
read_arabic('utf-8')

puts
puts 'Setting external and internal encoding on file'
read_arabic('utf-8:utf-8')

Encoding.default_external = Encoding.default_internal = 'utf-8'
puts
puts "Encoding.default_external: #{Encoding.default_external}"
puts "Encoding.default_internal: #{Encoding.default_internal}"

puts
puts 'Only setting external encoding on file'
read_arabic('utf-8')