Bug Description

The headers property in the DispatchOptions interface currently uses IncomingHttpHeaders. Since this property is used for outgoing HTTP requests, it would be more appropriate to type it as OutgoingHttpHeaders.

In addition to the typing issue, there is an inconsistency between the documentation and the actual TypeScript definitions in the code. According to the documentation (see screenshot below), the headers property is described as:

headers: UndiciHeaders | string[] (optional)

In contrast, the TypeScript definition for DispatchOptions in the codebase defines headers as:

headers?: IncomingHttpHeaders | string[] | Iterable<[string, string]> | null;

Additional Observation

While reviewing the typing of headers in DispatchOptions, I noticed a similar issue in the ConnectOptions interface, where the headers are also typed as IncomingHttpHeaders. This makes me wonder if there might be a broader issue with the typing of headers across the library. Perhaps there was an intention to use a generic type for headers, but it seems like only one of the possible types (likely IncomingHttpHeaders) has been applied throughout