[HttpKernel] fixed handling of X-Debug-Token stopwatch section in combination with HttpCache

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: #6227
License of the code: MIT

i wonder if openSection() shouldnt rather return a boolean if it was successful or not rather than throw an exception. but i guess its not worth it doing a BC break for this.