Ensure selectable container fix

This fixes the issue when inserting a node with a child node but no text content, a```BR``` is added to the wrong place.
E.g. ```<p><b></b></p>``` => ```<p><b></b><br></p>``` when it should be ```<p><b><br></b></p>```
`<p><b><br></b><i><br></i></p>` results in a visible line break.
`<p><b><br></b><i></i></p>` results in the cursor being in the B, and it's impossible to get into the I (you can delete it with Backspace though).

See:

http://jsbin.com/fuzaji/1/edit

I don't mind too much which one we do, we can keep the logic simple as it is and do option 1, but I think it would be nice to document that behaviour with a test so that it's clear what we expect it to do. Would that be ok?