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></b><i></i></p> (will become <p><b><br></b><i><br></i></p>) -- don't we want to only insert at most one BR per block-level element?

I'm not sure. If a BR was only inserted into the B, for example, you wouldn't be able to put your caret into the I.

If BRs were inserted in both (which they currently are), I'm not whether you will actually be able to place your caret in both B and I (perhaps using the arrow keys?).