Wiki macros executed before the content is evaluated breaks request-level sheets and the edit mode document sandboxing

How to reproduce:

• open http://localhost:8080/xwiki/bin/edit/Blog/BlogIntroduction?sheet=Sandbox.WebHome – it uses Sandbox.WebHome as the display sheet
• create an UIX, with org.xwiki.platform.template.header.after as the extension point, and containing a wiki macro, such as {{spaces/}} ({{menu}} is a more realistic example, but that requires installing the menu extension)
• reload http://localhost:8080/xwiki/bin/edit/Blog/BlogIntroduction?sheet=Sandbox.WebHome – the sheet parameter is now ignored