Spans stripped from source

[Matthew Leffler]

Steps to Reproduce:

1. ​http://ckeditor.com/demo
2. In source view clear the source and enter the following:

<p>Text<span class="whatever"> text</span></p>

<p>Copy me</p>

3. Switch to html and back to source view.

Expected:

The <span> should still be there.

Actual:

The <span> is removed.

Notes:

Is this desired behavior? This "works" as I expect it to in 4.0.1.

[Jakub Ś]

Please see - ​http://ckeditor.com/blog/CKEditor-4.1-RC-Released and docs about Advanced Content Filter(ACF) ​http://docs.ckeditor.com/#!/guide/dev_advanced_content_filter, ​http://docs.ckeditor.com/#!/guide/plugin_sdk_integration_with_acf