fix for undefined scrollbar style

src/js/easymde.js

@@ -2040,7 +2040,7 @@ EasyMDE.prototype.render = function (el) {
         allowDropFileTypes: ['text/plain'],
         placeholder: options.placeholder || el.getAttribute('placeholder') || '',
         styleSelectedText: (options.styleSelectedText != undefined) ? options.styleSelectedText : !isMobile(),
-        scrollbarStyle: options.scrollbarStyle,
+        scrollbarStyle: (options.scrollbarStyle != undefined) ? options.scrollbarStyle : 'native',
         configureMouse: configureMouse,
         inputStyle: (options.inputStyle != undefined) ? options.inputStyle : isMobile() ? 'contenteditable' : 'textarea',
         spellcheck: (options.nativeSpellcheck != undefined) ? options.nativeSpellcheck : true,