[fix] param serialization

package.json

@@ -1,6 +1,6 @@
 {
   "name": "simple-jsonp-promise",
-  "version": "1.0.5",
+  "version": "1.0.6",
   "description": "",
   "main": "build/index.js",
   "scripts": {

src/index.js

@@ -39,7 +39,7 @@ function jsonp( url, options ) {
     }
 
     function serialize( params ) {
-        let param = '';
+        let param = '?';
         for ( let key in params ) {
             if ( params.hasOwnProperty( key ) ) {
                 param += `&${key}=${encodeURIComponent( params[ key ] )}`;