Use native String#capitalize

activesupport/lib/active_support/multibyte/chars.rb

@@ -120,13 +120,6 @@ module ActiveSupport #:nodoc:
         slice(0...translate_offset(limit))
       end
 
-      # Converts the first character to uppercase and the remainder to lowercase.
-      #
-      #  'über'.mb_chars.capitalize.to_s # => "Über"
-      def capitalize
-        (slice(0) || chars("")).upcase + (slice(1..-1) || chars("")).downcase
-      end
-
       # Capitalizes the first letter of every word, when possible.
       #
       #   "ÉL QUE SE ENTERÓ".mb_chars.titleize.to_s    # => "Él Que Se Enteró"
@@ -184,7 +177,7 @@ module ActiveSupport #:nodoc:
         to_s.as_json(options)
       end
 
-      %w(capitalize reverse tidy_bytes).each do |method|
+      %w(reverse tidy_bytes).each do |method|
         define_method("#{method}!") do |*args|
           @wrapped_string = send(method, *args).to_s
           self

activesupport/test/multibyte_chars_test.rb

@@ -477,7 +477,7 @@ class MultibyteCharsUTF8BehaviourTest < ActiveSupport::TestCase
 
   def test_method_works_for_proxyed_methods
     assert_equal "ll", "hello".mb_chars.method(:slice).call(2..3) # Defined on Chars
-    chars = "hello".mb_chars
+    chars = +"hello".mb_chars
     assert_equal "Hello", chars.method(:capitalize!).call # Defined on Chars
     assert_equal "Hello", chars
     assert_equal "jello", "hello".mb_chars.method(:gsub).call(/h/, "j") # Defined on String