Use a more flexible type for hhtp headers signature

[axelsrz]

The headers property return type is declared as Dict[str, str], but in real HTTP requests, headers can have multiple values for the same key. Consider using Dict[str, Union[str, List[str]]] or a more flexible type like Mapping[str, str] that can accommodate framework-specific header types (e.g., aiohttp.multidict.CIMultiDictProxy, fastapi.datastructures.Headers).

Originally posted by @Copilot in #221 (comment)