In page parser, lex only if not already lexed

lib/parse/page.js

@@ -39,7 +39,8 @@ function render(section, _options) {
 function parsePage(src, options) {
     options = options || {};
 
-    return lex(src)
+    // Lex if not already lexed
+    return (_.isArray(src) ? src : lex(src))
     .map(function(section) {
         // Generate a uniqueId to identify this section in our code
         var id = _.uniqueId('gitbook_');