omit empty values with Image API clients

[kashitaka]

Issue

HI, I came across this project and I tried to use Image APIs. Then, I found that some optional parameters must be specify to call APIs, or it returns 400 error.

For example, according to the API specification of Create image variation API, the parameters n, response_format, and size are optional, and if those parameters aren't specified, the API applies default values.
However, in go-openai client, if those parameter aren't specified explicitly, it sends zero values and gets 400 response.

if I make a request like this:

	resp, err := client.CreateVariImage(
		context.Background(), openai.ImageVariRequest{
			Image: file,
		})

then, I got a response like this:

error, status code: 400, message: 0 is less than the minimum of 1 - 'n'

because client actually send a request like this:

curl https://api.openai.com/v1/images/variations \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -F image="foo.png" \
  -F n=0 \  # <- zero value for int
  -F size="" # <- zero value for string

Although the request structs have fields with json struct tag and seemingly intend to omitempty, actually this omitempty doesn't work because those API accept multipart/form-data so request aren't marshaled to json.

Describe the change

Then, I made image API client work even if optional parameters aren't set in request structs.

Provide OpenAI documentation link

• Create image variation

Describe your solution

I just wrote if to check zero values. This change will work same as omitempty. As discussed #9 , it cannot express the difference between zero value and intended zero or empty string though. Another alternative might be using a null structs like NullInt or NullString but it would make breaking changes though.

If my changes would be acceptable, I'd make changes in other image API clients that request with multipart/form-data format and write tests. If it was already argued or you don't need this changes, please close it. Thanks😃