allow to inject browser-sync client.js in custom middlewares

This small fix allows us to use any type of express/connect middlewares and does not break insertion browser-sync script.

Releted issue: https://github.com/shakyShane/browser-sync/issues/217

My use case:
```js
gulp.task('browser-sync', function() {
  var browserSync = require('browser-sync');
  var app = require('express')();

  app.set('view engine', 'jade');
  app.use(require('./mycustomrouter'));
  //file: mycustomrouter.js
  //var router = require('express').Router();
  //router.route('/my/custom/route')
  //  .get(function(req, res) {
  //    res.render('template_name', {data: customLocals);
  //  });
  //module.exports = router;

  browserSync({
    startPath: '/my/custom/route',
    server: {
      baseDir: './dist',
      middleware: app
    }
  });
});
```

This fix allow us to use `browser-sync` withous any script-injection hacks=)