[2.2] Pass ESI header to subrequests
merged branch mweimerskirch/patch-13 (PR #7253)

This PR was squashed before being merged into the 2.2 branch (closes #7253).

Discussion
----------

[2.2] Pass ESI header to subrequests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7252
| License       | MIT

Commits
-------

af819a7 [2.2] Pass ESI header to subrequests