[HttpFoundation] Default request `_format` should be aware of HTTP Accept header

Bug fix: no
Feature addition: yes
Backwards compatibility break: yes (?)
Symfony2 tests pass: yes
Fixes the following tickets: #4567

Possible __BC break__ would be in really edge case that is user sets `$default` format and expects it will be returned but now if `Accept` header is set, it has priority over default value.

> __Note:__ Segfault at Travis-CI is because of some latest changes merged to master, it was passing correctly last days.