Implement Request::time()

Back in the day, this was already merged in the L3 repo (https://github.com/laravel/laravel/pull/933).

This time, it is even the correct implementation (as suggested in https://github.com/laravel/laravel/pull/1057).

#### Two small questions for Taylor

1. Do you think it is fine to rely on the Symfony Request class definitely setting the time (see [second commit](https://github.com/franzliedke/http/commit/0a3866df709f8fd31dc3d0526c5560e7deeb97d2))?
2. Would you test this kind of behavior? I added a test, but I wondered whether this is too trivial, as this is basically a fancy setter (you didn't test the `ajax()` method either).