Pressing bold in middle of bold line unbolds entire line

[highjinx_53]

Steps to reproduce:

• Open replace by class example.
• Start with the following source:

  <p>
  	<b>This is a ^bold line.<br />
  	</b></p>
  

• Press the bold button
• Expected result:

  <p>
  	<b>This is a </b>^<b>bold line.<br />
  	</b></p>
  

• Encountered result:

  <p>
  	This is a bold line.<br />
  	</p>
  

Both Microsoft Word and Google Docs work correctly. Additionally, when doing the opposite (going from normal to bold), CKEditor works correctly.

This can be reproduced in any of the nightly samples.

[highjinx_53]

This is also an issue with italic and underline.

[highjinx_53]

I've attached a file which fixes this issue.

[garry.yao]

Hi highjinx_53, it's a good catch, a similar fix is provided at #2976.