use php://input if body is null

Use php://input if the body is null as [noted](https://github.com/codeguy/Slim/commit/1c4a990dc834728e1ad5f2d597071d9700504e7d#commitcomment-6083981) by @svemir. This was removed by mistake in the guzzle update pull request.

Some tests where added to make sure the read-only stream is used when the body is null.