Allow using Rails button helper with f.button :button. Closes #449

Make sure to just alias button when it is available, to avoid errors
with Rails versions prior to 3.2.

lib/simple_form/form_builder.rb

@@ -203,8 +203,12 @@ module SimpleForm
     #     f.button :submit
     #   end
     #
-    # It just acts as a proxy to method name given.
+    # It just acts as a proxy to method name given. We also alias original Rails
+    # button implementation (3.2 forward (to delegate to the original when
+    # calling `f.button :button`.
     #
+    # TODO: remove if condition when supporting only Rails 3.2 forward.
+    alias_method :button_button, :button if method_defined?(:button)
     def button(type, *args, &block)
       options = args.extract_options!
       options[:class] = [SimpleForm.button_class, options[:class]].compact

test/form_builder/button_test.rb

@@ -25,4 +25,11 @@ class ButtonTest < ActionView::TestCase
       assert_select 'form input.btn[type=submit][value=Save Post]'
     end
   end
+
+  if ActionView::Helpers::FormBuilder.method_defined?(:button)
+    test "allows to use Rails button helper when available" do
+      with_button_for :post, :button, 'Save!'
+      assert_select 'form button.button[type=submit]', 'Save!'
+    end
+  end
 end