Ensure correct XHR encoding for multipart/form-data forms (#174)

Fixes #168

lib/send-request.js

@@ -68,7 +68,7 @@ module.exports = function(location, options, callback) {
   request.setRequestHeader("X-PJAX-Selectors", JSON.stringify(options.selectors))
 
   // Send the proper header information for POST forms
-  if (requestPayload && requestMethod === "POST") {
+  if (requestPayload && requestMethod === "POST" && !formData) {
     request.setRequestHeader("Content-Type", "application/x-www-form-urlencoded")
   }