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

[beddows]

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.

[derenrich]

I'm getting a similar issue with this https://raw.githubusercontent.com/wikimedia/mediawiki-extensions-Wikibase/e45d4f725e25a95ec2b034238fd3736f19053a16/repo/rest-api/specs/openapi.json (moving this comment from #924).

In this case it cries about:

Union[Unset, List[str]] is not allowed in header

where I'm assuming the issue is it doesn't understand how to map a list of strings to a string. Looking at the swagger endpoint for this API (https://doc.wikimedia.org/Wikibase/master/js/rest-api/#/items/getItem) it seems they want us to encode a list of strings as If-Match: "1276703231","1276705620"' . But I'm not sure if that's a standard openapi thing or if that's just some hack. If it's the latter it should probably have built in support.