setRequestHeader can XMLHttpRequest after OPENing a connection

i got weird error messages after i applied your changes. 

in safari: 
  `An attempt was made to use an object that is not, or is no longer, usable.` 
chrome was i bit more helpful: 
  `Failed to execute 'setRequestHeader' on 'XMLHttpRequest': the object's state must be OPENED.`

But thanks. that makes it possible to refer to `request.xhr?` in rack based applications.