[BUG FIX] Encode content property with nil value when responding to function calling

[ysak-y]

What

Encode content property with nil value in Chat struct build Chat instance for responding to function calling result.

Why

We must send two chat instances when sending function calling result to OpenAI.
Ref: Function calling example section of https://openai.com/blog/function-calling-and-other-api-updates

Chat(role: .assistant, content: nil, functionCall: functionCall),
Chat(role: .function, content: functionContent, name: functionName)

Most importantly, we must encode content property with nil value for the first one. But current encoder behavior removes content property if value is nil which is the default behavior of JSONEncoder.

Actually, if I send above messages to chat endpoint, then I get 400 error as follows because payload doesn't include content property.

{
  "error": {
    "message": "'content' is a required property - 'messages.1'",
    "type": "invalid_request_error",
    "param": null,
    "code": null
  }
}

Payload doesn't have content property actually because it is nil value.
{"role":"assistant","function_call":{"name":"add","arguments":"{\"a\": \"1\", \"b\": \"1\"}"}}

So I want to fix this behavior.

Affected Areas

I modified encoding behavior of Chat struct not to remove content property even if it is nil but has functionCall value. by overriding encode(to encoder:) method. Other properties would be encoded same as originally which means it would be included if it has the value, otherwise, not.

[Krivoblotsky]

Looks good to me, thanks!