use while instead of #map

Callbacks array could be modified during #map. This cause errors in
jruby and rubinius.

Author: lest

lib/nested_form/view_helper.rb

@@ -33,8 +33,9 @@ def after_nested_form(association, &block)
     private
       def after_nested_form_callbacks
         @after_nested_form_callbacks ||= []
-        fields = @after_nested_form_callbacks.map do |callback|
-          callback.call
+        fields = []
+        while callback = @after_nested_form_callbacks.shift
+          fields << callback.call
         end
         fields.join(" ").html_safe
       end