ability to set image as input type in submit_to_remote

1. I noticed that the method call, to submit_to_remote, given below, while seemingly valid, wasn't working since the method
   button_to_function in actionpack/lib/action_view/helpers/ javascript_helper.rb was forcing the type to button.

<%=
   submit_to_remote(
     'update_resource_img',
     'Update Resource 2',
     :url => { :controller => 'hello', :action => 'update' },
     :before => "$('loading').style.display = 'block'",
     :after => "return false",
     :html => { :src => '/images/button.gif', :type => 'image' }
   )
%>

1. It is my opinion that button_to_remote should allow the input type to be either a button or an image since the w3c page on form input control types states that the input type image "Creates a graphical submit button." - http://www.w3.org/TR/html401/interact/forms.html#h-17.4.1

2. Hence, I have created the attached patch, that adds two test cases to javascript_helper_test.rb and one test case to prototype_helper_test.rb to illustrate the issue at hand and tests the modifications that I have placed in javascript_helper.rb