fixed locale of sub-requests when explicitely set by the developer (refs #8821)
Fixed issue #6932 - Inconsistent locale handling in subrequests

The fix consists in passing the locale in the controller reference,
inside the routablefragment renderer.
added a functional test for locale handling in sub-requests
[HttpKernel] added a comment to warn about possible inconsistencies
fixed request format when forwarding a request
merged branch fabpot/fragment-locale (PR #8822)

This PR was merged into the 2.2 branch.

Discussion
----------

fixed locale management in sub-requests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6932, #5532, #8604
| License       | MIT
| Doc PR        | n/a

Same fix as #8821 but for `_locale`, and a rebase of #8604.

Commits
-------

7117328 [HttpKernel] added a comment to warn about possible inconsistencies
c4636e1 added a functional test for locale handling in sub-requests
05fdb12 Fixed issue #6932 - Inconsistent locale handling in subrequests
b3c3159 fixed locale of sub-requests when explicitely set by the developer (refs #8821)
merged branch fabpot/format-forward (PR #8829)

This PR was merged into the 2.2 branch.

Discussion
----------

fixed request format when forwarding a request

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6932
| License       | MIT
| Doc PR        | n/a

When calling forward() in a controller, the format is not the same as for the master request.

Commits
-------

7e87eb1 fixed request format when forwarding a request