Faker::Internet.user_name can't handle UTF-8 arguments (faker-ruby#1421)

* test: covered bug

* fix: passed

* fix: typo

* Minor fix

* Fix test_username_with_utf_8_arg - Faker::Internet.user_name

* Add comment above test_username_with_utf_8_arg

* Update test_faker_internet.rb

lib/faker/internet.rb

@@ -21,7 +21,7 @@ def safe_email(name = nil)
 
       def username(specifier = nil, separators = %w[. _])
         with_locale(:en) do
-          return shuffle(specifier.scan(/\w+/)).join(sample(separators)).downcase if specifier.respond_to?(:scan)
+          return shuffle(specifier.scan(/[[:word:]]+/)).join(sample(separators)).downcase if specifier.respond_to?(:scan)
 
           if specifier.is_a?(Integer)
             # If specifier is Integer and has large value, Argument error exception is raised to overcome memory full error

test/test_faker_internet.rb

@@ -48,6 +48,15 @@ def test_username_with_integer_arg
     end
   end
 
+  def test_username_with_utf_8_arg
+    # RUBY_VERSION < '2.4.0' is not able to downcase or upcase non-ascii strings
+    if RUBY_VERSION < '2.4.0'
+      assert @tester.username('Łucja').match('Łucja')
+    else
+      assert @tester.username('Łucja').match('łucja')
+    end
+  end
+
   def test_username_with_very_large_integer_arg
     exception = assert_raises(ArgumentError) { @tester.username(10_000_000) }
     assert_equal('Given argument is too large', exception.message)