Block-level html in shortcodes content gets parsed and styled in the visual editor 

# Description:
SH passes unescaped html to the tiny mce editor. The tags are getting
parsed and styled by the user-agent in some instances. The reset
specifies that children of pre should inherit some style from pre.

## Steps to reproduce:
1. Create a post
2. In the text tab of tiny mce add a SH shortcode with an h1 (or any block-level tag) in it.
3. Save the post
4. After page reload, switch to the visual tab

_Example short code_
    ```[html]<h1>This is an h1</h1>[html]```

## Expected output:
The user should be able to switch tiny mce modes (text/visual) and the content of the shortcode will consistently be styled pre in visual mode

## Actual Output:
Since SH passes unescaped html to the editor, the html gets parsed by the browser applying user-agent styles and theme editor styles to the h1 tag