fix bugs (fullscreen)

src/js/editormd.js

@@ -740,7 +740,7 @@
          */
         
         hideInfoDialog : function() {
-            $("html,body").css("overflow-x", null);
+            $("html,body").css("overflow-x", "");
             this.editor.find("." + this.classPrefix + "dialog-info").fadeOut();
 
             return this;
@@ -1318,7 +1318,7 @@
             
             this.toolbar.find(".fa[name=fullscreen]").parent().removeClass("active"); 
 
-            $("html,body").css("overflow", null);
+            $("html,body").css("overflow", "");
 
             editor.css({
                 position : "",