`format: true` does not override existing format constraints.

Closes #9466.

Passing `format: true` used to override the constraints: { format: /json/ }
with `/.+/`. This patch only sets the format if there is no constraint present.
Merge pull request #9469 from senny/9466_format_enforcing_routes

`format: true` does not override existing format constraints.
Remove redundant method
Minor changelog improvement [ci skip]