pallets/werkzeug#2352

* Add failing tests

* Use for else construct

* openapi: remove body preprocessing

Body is already validated using jsonschema. There was also some type
casting but it was wrong: e.g. not recurring deeply into dicts and lists,
relying on existence of "type" in schema (which is not there e.g. if
oneOf is used). Anyway, the only reason why types should be casted is
converting integer values to float if the type is number. But this is in
most cases irrelevant.

Added an example, which did not work before this commit (echoed `{}`)
e.g. for
```
curl localhost:8080/api/foo -H 'content-type: application/json' -d
'{"foo": 1}'
```
but now the example works (echoes `{"foo": 1}`).

* test with oneOf in the requestBody

* remove oneof examples: superseded by tests

Co-authored-by: Pavol Vargovcik <pavol.vargovcik@kiwi.com>