Multibrowser test with capybara using Rake(if set two browser only work the last)

Rakefile

require 'rubygems'
require 'bundler'

Bundler.setup(:default)


desc 'Run specs multibrowser test'
task :spec, [:browser] do |t,args|
  Bundler.setup(:test)
  require 'rspec/core/rake_task'
  ENV['TEST_BROWSER']=args.browser
  
  RSpec::Core::RakeTask.new('spec') do |t|
    t.pattern = FileList['spec/spec_base.rb'] 
  end

end

spec_base.rb

require 'rspec'
require 'rack/test'
require 'capybara'
 
include Capybara::DSL

browsers=ENV["TEST_BROWSER"].to_s.split
 
Capybara.default_driver = :selenium #this is needed to fix a bug when browser is null
 
browsers.each{|browser|
  puts "#{browser}"
  ENV['CURRENT_BROWSER']=browser
  unless browser.nil?
    Capybara.register_driver :selenium do |app|   
      case browser
        when "-p"
          require 'capybara/poltergeist'
          Capybara.current_driver = :poltergeist
        when "-c"
          Capybara::Selenium::Driver.new(app, :browser => :chrome)
        when "-i"
          Capybara::Selenium::Driver.new(app, :browser => :internet_explorer)
        when "-s"
          Capybara::Selenium::Driver.new(app, :browser => :safari)
        when "-f"
          Capybara::Selenium::Driver.new(app, :browser => :firefox)
          
      end
      
    end 
  end
  #require all files or the file that we want test
  if ENV['TEST'].nil?
    Dir[ "#{File.dirname(__FILE__)}/*_spec.rb" ].sort.each do |path|
      puts path
      load(path)
    end
  else
    load "spec/#{ENV['TEST']}"
  end 
}

RSpec.configure do |conf|
  conf.include Rack::Test::Methods
end
 
def app
end