Fix AS::MB::Chars#+ to not alter self [#4646 state:resolved]

Signed-off-by: José Valim <jose.valim@gmail.com>

activesupport/lib/active_support/multibyte/chars.rb

@@ -105,7 +105,7 @@ def self.wants?(string)
         # Example:
         #   ('Café'.mb_chars + ' périferôl').to_s #=> "Café périferôl"
         def +(other)
-          self << other
+          chars(@wrapped_string + other)
         end
 
         # Like <tt>String#=~</tt> only it returns the character offset (in codepoints) instead of the byte offset.

activesupport/test/multibyte_chars_test.rb

@@ -49,13 +49,15 @@ def test_forwarded_method_with_non_string_result_should_be_returned_vertabim
   end
 
   def test_should_concatenate
-    assert_equal 'ab', 'a'.mb_chars + 'b'
-    assert_equal 'ab', 'a' + 'b'.mb_chars
-    assert_equal 'ab', 'a'.mb_chars + 'b'.mb_chars
+    mb_a = 'a'.mb_chars
+    mb_b = 'b'.mb_chars
+    assert_equal 'ab', mb_a + 'b'
+    assert_equal 'ab', 'a' + mb_b
+    assert_equal 'ab', mb_a + mb_b
 
-    assert_equal 'ab', 'a'.mb_chars << 'b'
-    assert_equal 'ab', 'a' << 'b'.mb_chars
-    assert_equal 'ab', 'a'.mb_chars << 'b'.mb_chars
+    assert_equal 'ab', mb_a << 'b'
+    assert_equal 'ab', 'a' << mb_b
+    assert_equal 'abb', mb_a << mb_b
   end
 
   def test_consumes_utf8_strings