test and fix for live reload sometimes missing files

Test and fix for https://github.com/gruntjs/grunt-contrib-watch/issues/191

    assertWatch([function() {
      request(9876, function(data) {
        resultData += data;
        grunt.file.write(path.join(cwd, 'lib', 'one.js'), 'var one = true;');
        grunt.file.write(path.join(cwd, 'lib', 'two.js'), 'var two = true;');
      });
    }

But if *feels* race condition-y to me. 

The test seems to fail for me every time without the fix in `watch.js` and succeeds every time with the fix in `watch.js` so, if nothing else, it's hopefully a good start. And now my grunt task is working, woot woot.