Config\Extensions\NetteExtension: header() called only if !headers_sent()

If there is the **security.frames** configuration option set to any value except **TRUE** the `header()` function call is added to the `SystemContainer::initialize()` method.

If there is some output before the container's initialize method is called (nothing unusual in scripts, tests, etc.) this causes `Nette\FatalErrorException` with the message **"Cannot modify header information - headers already sent by (output started at ...)"**

Perhaps some test of `$consoleMode` or `headers_sent()` should be added to the definition.

Now it is needed to use this `config.neon` file for tests to disable `header()` function call addition:

```
nette:
    security:
        frames: on
```