Validate body of MockContext

[Resteklicken]

Is your feature request related to a problem? Please describe.
In my tests I use fuego.MockContext. The Body() method just returns mockCtx.RequestBody that can be set during fuego.NewMockContext(), skipping any validation that would happen with a real request context.

Describe the solution you'd like
I would like the Body() method to perform proper validation on mockCtx.request.Body if mockCtx.RequestBody is unset.

Describe alternatives you've considered
I have found no way to test validation via the route itself.

Additional context
It might be necessary to address #568 first to be able to actually set mockCtx.request.

[dylanhitt]

What type of validation would you expect on mockCtx.request.Body?

[Resteklicken]

What type of validation would you expect on mockCtx.request.Body?

Hey, I would like it to jump through the same hoops as the body in a normal context.

This will become relevant once #616 goes through. How it would interact with the "shortcut" mockCtx.RequestBody is an open question, since that field can diverge from mockCtx.request.Body. People might use one but not the other. Maybe the method could check whether mockCtx.request.Body is set and otherwise return mockCtx.RequestBody without validation (current behaviour)?

This might make the MockContext API more complex than it needs to be. On the other hand, it would enable provoking validation errors in tests.