Fix custom fields application

xhr was not in scope. xdr was intended. This is fixed.
Using $.extend() means we do not have to specify each field.
Renamed xhrFields to xdrFields. As this is what they are.
Fomerly, property names were unnecessarily translated:
	xhrFields.error -> xhr.onerror
	xhrFields.timeout -> xhr.ontimeout
Now there is no translation, use the exact names:
	xdrFields.onerror -> xdr.onerror
	xdrFields.ontimeout -> xdr.ontimeout

jquery.iecors.js

@@ -25,9 +25,8 @@
           };
 
           // Apply custom fields if provided
-          if ( s.xhrFields ) {
-            xhr.onerror = s.xhrFields.error;
-            xhr.ontimeout = s.xhrFields.timeout;
+          if ( s.xdrFields ) {
+            $.extend(xdr, s.xdrFields);
           }
 
           xdr.open( s.type, s.url );