fix params encoding in sys.Http - StringTools.urlEncode(null) is not specified (closes #7099)

std/sys/Http.hx

@@ -154,7 +154,7 @@ class Http extends haxe.http.HttpBase {
 					uri = "";
 				else
 					uri += "&";
-				uri += StringTools.urlEncode(p.name)+"="+StringTools.urlEncode(p.value);
+				uri += StringTools.urlEncode(p.name)+"="+StringTools.urlEncode('${p.value}');
 			}
 		}