Simplify handling of defaults/options in button_tag

There's no need to rely on Active Support's Hash#reverse_merge for
simple cases with default values, since we can just merge from the
default rather than reverse merge from the options.

This also avoids the creation of one extra hash object by moving to a
Hash#merge! call.

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -469,16 +469,13 @@ def submit_tag(value = "Save changes", options = {})
       #   # => <button data-disable-with="Please wait..." name="button" type="submit">Checkout</button>
       #
       def button_tag(content_or_options = nil, options = nil, &block)
-        default_options = { 'name' => 'button', 'type' => 'submit' }
-
         if content_or_options.is_a? Hash
           options = content_or_options
         else
           options ||= {}
         end
 
-        options = options.stringify_keys
-        options = options.reverse_merge default_options
+        options = { 'name' => 'button', 'type' => 'submit' }.merge!(options.stringify_keys)
 
         if block_given?
           content_tag :button, options, &block