Sets _format attribute only if it wasn't set previously by the user.

Fixes #8787
fixed request format of sub-requests when explicitely set by the developer (closes #8787)
merged branch fabpot/inline-fragment-format (PR #8821)

This PR was merged into the 2.2 branch.

Discussion
----------

Sets _format attribute only if it wasn't set previously by the user

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8787, #8791
| License       | MIT
| Doc PR        |

Commits
-------

9bb7a3d fixed request format of sub-requests when explicitely set by the developer (closes #8787)
fa35597 Sets _format attribute only if it wasn't set previously by the user.