Renamed IsJsonOnly method (#523)

## Changes
Renamed IsJsonOnly method

## Tests
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How is this tested? Please see the checklist below and also describe any
other relevant tests
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- [x] `make test` passing
- [x] `make fmt` applied
- [ ] ~relevant integration tests applied~

openapi/code/method.go

@@ -154,7 +154,7 @@ func (m *Method) IsCrudCreate() bool {
 	return m.operation.Crud == "create"
 }
 
-func (m *Method) JsonOnly() bool {
+func (m *Method) IsJsonOnly() bool {
 	return m.operation.JsonOnly
 }