fix: stringify content in parser call

updated to work with newer htmlparser2 api

Author: cossssmin

src/html.js

@@ -168,7 +168,7 @@ module.exports = function( options, callback )
                 return callback( null );
             },{ normalizeWhitespace: true } );
             var parser = new htmlparser.Parser( handler, { xmlMode: true } );
-            parser.write( content );
+            parser.write( content.toString() );
             parser.done();
         } );
     };