& get's escaped multiple times in TinyMCE 4

When using TinyMCE 4 a problem can occur with the escaping of `&` characters.

There is a blind search and replace of `&` to `&`.
In the TinyMCE javascript there is unescaping and escaping for `pre_wpautop` and `wpautop` which should avoid this problem.

The solution which worked best for me is to add an additional replace to make sure there are no `&` occurences in the shortcode content.

No file or test is named. Start by tracing the blind ampersand-to-ampersand-entity replacement and the TinyMCE pre_wpautop and wpautop escaping paths; verify that shortcode content no longer produces amp;amp; occurrences.

Written by the indexing model from the issue text.