Add CHANGELOG entry for #8108 on master too.

[ci skip]

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Fix input name when `:multiple => true` and `:index` are set.
+
+    Before:
+
+        check_box("post", "comment_ids", { :multiple => true, :index => "foo" }, 1)
+        #=> <input name=\"post[foo][comment_ids]\" type=\"hidden\" value=\"0\" /><input id=\"post_foo_comment_ids_1\" name=\"post[foo][comment_ids]\" type=\"checkbox\" value=\"1\" />
+
+    After:
+
+        check_box("post", "comment_ids", { :multiple => true, :index => "foo" }, 1)
+        #=> <input name=\"post[foo][comment_ids][]\" type=\"hidden\" value=\"0\" /><input id=\"post_foo_comment_ids_1\" name=\"post[foo][comment_ids][]\" type=\"checkbox\" value=\"1\" />
+
+    Fix #8108
+
+    *Daniel Fox, Grant Hutchins & Trace Wax*
+
 *   Clear url helpers when reloading routes.
 
     *Santiago Pastorino*