[HttpFoundation] Fixing broken http auth digest in some circumstances.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | can be refered in issue #1813
| License       | MIT
| Doc PR        | n/a

With some apache + php-fpm setup we need to set ```PHP_AUTH_DIGEST``` value if not already setted in GLOBAL vars.

Added some unit tests too.