[htmlmixed mode] Strip whitespace from attribute values

Closes #4147

mode/htmlmixed/htmlmixed.js

@@ -46,7 +46,7 @@
 
   function getAttrValue(text, attr) {
     var match = text.match(getAttrRegexp(attr))
-    return match ? match[2] : ""
+    return match ? /^\s*(.*?)\s*$/.exec(match[2])[1] : ""
   }
 
   function getTagRegexp(tagName, anchored) {