Removed the single space character for Safari

~I'm still cleaning up the tests, but I want to get this out so people can help me test it~

Once upon a time, a single space character was added to `render nothing: true` (https://github.com/rails/rails/commit/cb0f8fda9652c4d24d04693bdb82cecd3b067e5c, https://github.com/rails/rails/commit/807df4fcf021fc4d15972aa1c17ba7398d43ab0d) to fix a bug in [Safari](https://bugs.webkit.org/show_bug.cgi?id=5924). (Thank you @matthewd for hunting this down.)

Since this bug has been fixed in Safari itself a long time ago (based on the timestamps it should be fixed in Safari 2.x or perhaps 1.x in ~2005), it's probably a good time to remove this.

I wrote a simplified version of the [webkit test case](https://github.com/WebKit/webkit/blob/master/LayoutTests/http/tests/xmlhttprequest/zero-length-response.html) which you can access here:

- xhr async version: http://lit-anchorage-8910.herokuapp.com/async ([source](https://github.com/chancancode/zero-length/blob/master/app/views/blank/async.html))
- xhr sync version: http://lit-anchorage-8910.herokuapp.com/sync ([source](https://github.com/chancancode/zero-length/blob/master/app/views/blank/sync.html))

(The controller doesn't actually use `render nothing`, but it should have the [same effect](https://github.com/chancancode/zero-length/blob/master/app/controllers/blank_controller.rb#L10))

I tested this on a variety of browsers on [browser stack(http://www.browserstack.com/)], including Safari 4.0 and Firefox 3.0 and it seems to work fine.

Ideally, I'd like to see this test case fail on an older version of safari (you might need safari 2.0 or even 1.0). **If you have access to one of those super old browser, please help run the test case and let me know what happens** :smile:

Otherwise, I am fairly confident that this should be okay.

cc @dhh @jeremy @sikachu @matthewd

Fixes #14336