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apiextensions: fix object keys in fuzzer to exclude escape characters

Jsoniter in ConfigFastest mode does not support escape characters in object keys. Hence, we have to fix this after the fuzzer chose invalid keys.

This might be only an intermediate fix if we decide to accept arbitrary object keys again. But for now, with the choice of `ConfigFastest` (https://github.com/json-iterator/go/blob/f1258b01aa14930fff5c6b9b09fe9b4614359472/feature_config.go#L66) this change is necessary.