Is your feature request related to a problem? Please describe.

#11744

Just like issue above, I also want to append header.

const init = {
    method: context.method,
    headers: headers,
    body,
    credentials: this.configuration.credentials,
    ...initOverrides
};

But {X, ...Y} is one level merging, not deep merging.

When initOverrides has key headers, prev key headers is gone.

So, current implement can't append header.

Describe the solution you'd like

Current signature

private createFetchParams(context: RequestOpts, initOverrides?: RequestInit)

Merging using {X, ...Y}

REQ

private createFetchParams(
    context: RequestOpts,
    initOverrides?: RequestInit | ((context: RequestOpts) => RequestInit)
)

Merging using {X, ...Y(X) }

Describe alternatives you've considered

Convert {X, ...Y} to some deep merging library fn(X, Y).

But consider openapi-generator target many language, I prefer former to decrease you guys maintenance burden.

Additional context

You guys are awesome. :)