String-only metrics need to be returned as JSON or they won't be processed correctly

Author: pmeenan

dist/doctype.js

@@ -13,4 +13,4 @@ if ( dt ) {
   }
 }
 /* eslint-disable-next-line no-control-regex */
-return sDoctype.replace(/[\x00-\x1F\x80-\xFF]/g, "").toString();
+return JSON.stringify(sDoctype.replace(/[\x00-\x1F\x80-\xFF]/g, "").toString());

dist/lib-detector-version.js

@@ -2,4 +2,4 @@
  * IMPORTANT: Do not modify this file directly! It is generated by
  *   bin/library-detector.js
  */
-return '6.1.0';
+return JSON.stringify('6.1.0');