Fix default render option

This should make it compatible with newer React versions again.

Closes #15

lib/options.js

@@ -34,13 +34,9 @@ function defaultGetSyntax(block) {
     @return {React.Element}
 */
 function defaultRender(props) {
-    props = extend({}, props, {
-        className: 'prism-token token ' + props.type
-    });
-
     return React.createElement(
       "span",
-      props,
+      { className: 'prism-token token ' + props.type },
       props.children
     );
 }