Union[Unset, datetime.date] is not allowed in header

Describe the bug
The parameter name='IF-MODIFIED-SINCE' produces an error 'Union[Unset, datetime.date] is not allowed in header', which prevents the endpoint from being generated.

OpenAPI Spec File
https://docs.developers.clio.com/openapi.json

Desktop (please complete the following information):

• OS: macOS 13.4.1
• Python Version: 3.12
• openapi-python-client version: 0.15.2

Additional context
The parameter schema is:
"schema": {
"type": "string", (the format in "YYYY-MM-DD")
"format": "date" (the format defined by full-date in RFC3339)
}
The client generator is attempting to map this schema to a Python Union type, the union of Unset and datetime.date, which is not allowed or fitting in a HTTP header. Maybe string dates need to be transformed to datetime objects and vice versa? Or the date could be converted to a string before setting the header? At least that's my limited understanding of the situation.

未指定 repository 檔案或測試。首先使用連結的 OpenAPI 規格重現產生流程，並追蹤 IF-MODIFIED-SINCE header 參數的處理；檢查字串日期在產生的 header 中如何表示。當 endpoint 能夠成功產生且不再出現 Union[Unset, datetime.date] 錯誤，並且日期 header 的表示形式已有測試時，即視為完成。