Simplify String#mb_chars and stop proxying #class

This behavior mattered under Ruby 1.8, but that doesn't matter now
that we don't support it.

In addition, we don't want to proxy the #class method. A test was added
to prevent against regressions.

activesupport/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   No longer proxy ActiveSupport::Multibyte#class. *Steve Klabnik*
+
 *   Deprecate `ActiveSupport::TestCase#pending` method, use `skip` from MiniTest instead. *Carlos Antonio da Silva*
 
 *   `XmlMini.with_backend` now may be safely used with threads:

activesupport/lib/active_support/core_ext/string/multibyte.rb

@@ -33,11 +33,7 @@ class String
   # For more information about the methods defined on the Chars proxy see ActiveSupport::Multibyte::Chars. For
   # information about how to change the default Multibyte behavior see ActiveSupport::Multibyte.
   def mb_chars
-    if ActiveSupport::Multibyte.proxy_class.consumes?(self)
-      ActiveSupport::Multibyte.proxy_class.new(self)
-    else
-      self
-    end
+    ActiveSupport::Multibyte.proxy_class.new(self)
   end
 
   def is_utf8?

activesupport/test/multibyte_chars_test.rb

@@ -47,7 +47,7 @@ def test_forwarded_bang_method_calls_should_return_nil_when_result_is_nil
   end
 
   def test_methods_are_forwarded_to_wrapped_string_for_byte_strings
-    assert_equal BYTE_STRING.class, BYTE_STRING.mb_chars.class
+    assert_equal BYTE_STRING.length, BYTE_STRING.mb_chars.length
   end
 
   def test_forwarded_method_with_non_string_result_should_be_returned_vertabim
@@ -673,6 +673,9 @@ def test_tidy_bytes_should_forcibly_tidy_bytes_if_specified
     assert_equal "𥤤", chars(byte_string).tidy_bytes(true)
   end
 
+  def test_class_is_not_forwarded
+    assert_equal BYTE_STRING.dup.mb_chars.class, ActiveSupport::Multibyte::Chars
+  end
 
   private