Fix associative container on Json values in UE4cpp

[roseatromero]

Similar to #17684

Fixes compiler error when using "uniqueItems": true, in the schema

11>JsonWriter.h(211): Error C2665 : 'TJsonWriter<TCHAR,TPrettyJsonPrintPolicy<CharType>>::WriteValueOnly': no overloaded function could convert all the argument types
        

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  Tagging @Kahncode for visibility

[Kahncode]

Looks good, thanks for the PR @roseatromero.

Did you have this locally for a long time? Your previous change would not have worked for Sets without it.

[roseatromero]

Looks good, thanks for the PR @roseatromero.

Did you have this locally for a long time? Your previous change would not have worked for Sets without it.

I could be wrong, but I believe it was working but only for serialization ,and not for deserialization (we were sending requests instead of receiving unique values as well).