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 文件或测试。首先使用链接的 OpenAPI 规范重现生成过程，并跟踪 IF-MODIFIED-SINCE header 参数的处理；检查字符串日期在生成的 header 中是如何表示的。当 endpoint 能够成功生成且不再出现 Union[Unset, datetime.date] 错误，并且日期 header 的表示形式已有测试时，即视为完成。