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.

No se nombran archivos del repositorio ni pruebas. Empieza reproduciendo la generación con la especificación OpenAPI enlazada y siguiendo el procesamiento del parámetro de cabecera IF-MODIFIED-SINCE; inspecciona cómo se representan las fechas como strings en las cabeceras generadas. Se considera terminado cuando el endpoint se genera correctamente sin el error Union[Unset, datetime.date], con una representación probada para la cabecera de fecha.