The macro content is preserved when switching macros even if the new macro doesn't support content

Follow this steps to reproduce:

• Edit a new page
• Insert an Info macro with some content (e.g. "test")
  • If you switch to source you should see:

    {{info}}
    test
    {{/info}}
    

• Edit the Info macro you just inserted and change it to the Page Tree.
  • If you switch to source you see:

    {{documentTree}}
    test
    {{/documentTree}}
    

That's not right since the Page Tree macro doesn't support content.