fix slim issue 552 (work around for bad sinatra behaviour)

* https://github.com/slim-template/slim/issues/552
* https://github.com/sinatra/sinatra/issues/939

* Sinatra should not pass the invalid option :outvar, it is called :buffer
in Slim/Temple.
* Sinatra should not set the buffer/outvar to an instance variable which
is overwritten for subtemplate rendering
* We print a warning now.