README

The first implementation was too long (8 LOCs) and referenced `options[:class]` 3 times (duplication).
After the refactoring, with two invocations and a single line of code the problem is solved.

layout_helper.rb

module LayoutHelper
  # ...

  # before
  def submit_button(content, options = {})
    options[:class] ||= ""
    options[:class] << " submit"
    options[:class].lstrip!
    options.stringify_keys!

    if disable_with = options.delete('disable_with')
      options['data-disable-with'] = disable_with
    end

    content_tag :button, content, options
  end 

  # after
  def submit_button(content, options = {})
    options.merge!(:class => "#{options[:class]} submit".lstrip)
    options.stringify_keys!

    if disable_with = options.delete('disable_with')
      options['data-disable-with'] = disable_with
    end

    content_tag :button, content, options
  end
end

layout_helper_spec.rb

require 'spec_helper'

describe LayoutHelper do
  # ...

  describe "#submit_button" do
    it "renders a button tag for form submission" do
      helper.submit_button("Create").should == %(<button class="submit">Create</button>)
      helper.submit_button("Create", :class        => "cool").should == %(<button class="cool submit">Create</button>)
      helper.submit_button("Create", :id           => "cool").should == %(<button class="submit" id="cool">Create</button>)
      helper.submit_button("Create", :disable_with => "Creating..").should == %(<button class="submit" data-disable-with="Creating..">Create</button>)
    end
  end
end