Add the option to use FormData to encode the form elements

If the form's enctype attribute is set to "multipart/form-data",
use FormData to encode the form's elements.

lib/send-request.js

@@ -51,6 +51,9 @@ module.exports = function(location, options, callback) {
         break
     }
   }
+  else if (requestOptions.formData) {
+    requestPayload = requestOptions.formData
+  }
 
   // Add a timestamp as part of the query string if cache busting is enabled
   if (options.cacheBust) {