Add default content-type for restjson

[DyfanJones]

Fixes #749

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (37d87ce) 84.91% compared to head (d6f1de7) 84.97%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #751 +/- ## ========================================== + Coverage 84.91% 84.97% +0.05% ========================================== Files 204 204 Lines 14986 15019 +33 ========================================== + Hits 12726 12762 +36 + Misses 2260 2257 -3 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.