Steps to reproduce:

1. Create item
2. Set type to HTTP agent
3. Set request body type to JSON / XML
4. Add data to the body that is not valid according to the selected type (not valid JSON / XML)

Result:

Zabbix will not allow saving such a configuration.

Expected:
It is expected that HTTP client will not validate data as it should be done by HTTP server. We could leave a notice that body may not be valid for the selected type, while still allowing to save the data.