Bold button become inactive on backspace

[benbro]

[Describe the issue]

Steps for Reproduction

1. Visit http://beta.quilljs.com/playground/
2. Click on the Bold (B) button
3. Type 'a'
4. Click 'Enter' to get a new line
5. Click Backspace to get back to the previous line

Expected behavior: [expected]
The Bold Button should be active

Actual behavior: [actual]
The Bold button is inactive. Typing another character is bold as expected.

Platforms:
Firefox 46 on Windows 7

Version: [version]
1.0-beta.4