Check for htmlExpr in unload() method

Based on a comment from @nominalaeon I noticed there was a oversight in
the unload method, any arrows which were set in the options as a jquery
selector-string were being removed from the DOM and never replaced in when
a slide was added/removed, or setOption was run, or the slides were
filtered.

Fixed.

slick/slick.js

@@ -2288,16 +2288,23 @@
         var _ = this;
 
         $('.slick-cloned', _.$slider).remove();
+
         if (_.$dots) {
             _.$dots.remove();
         }
-        if (_.$prevArrow && (typeof _.options.prevArrow !== 'object')) {
+
+        if (_.$prevArrow && _.htmlExpr.test(_.options.prevArrow)) {
             _.$prevArrow.remove();
         }
-        if (_.$nextArrow && (typeof _.options.nextArrow !== 'object')) {
+
+        if (_.$nextArrow && _.htmlExpr.test(_.options.nextArrow)) {
             _.$nextArrow.remove();
         }
-        _.$slides.removeClass('slick-slide slick-active slick-visible').attr("aria-hidden", "true").css('width', '');
+
+        _.$slides
+            .removeClass('slick-slide slick-active slick-visible')
+            .attr("aria-hidden", "true")
+            .css('width', '');
 
     };