Add unit test for bug: extra escaping of & in ' entity for apostrophe

When found in a replacement value, ' should be left untouched, not escaped
as ' which makes the entity visible instead of displaying an apostrophe
in a browser.

spec/_files/escaped.js

@@ -2,5 +2,6 @@ var escaped = {
   title: function() {
     return "Bear > Shark";
   },
+  subtitle: 'Rock 'n Roll',
   entities: """
 };

spec/_files/escaped.mustache

@@ -1,2 +1,3 @@
 <h1>{{title}}</h1>
+<h2>{{subtitle}}</h2>
 But not {{entities}}.

spec/_files/escaped.txt

@@ -1,2 +1,3 @@
 <h1>Bear &gt; Shark</h1>
+<h2>Rock &#39;n Roll</h2>
 But not &quot;.