fix request body type detection when using headers, cookies

[fedorg]

The library had trouble detecting the POST body payload types of my API. The argument was being typed as an empty object, and preventing me from sending any payloads. I had to use hacks such as these to make things work:

type CreateStationArg = components['schemas']['CreateStation'] // type hack
export const CreateStation = (arg: CreateStationArg, init?: RequestInit) => fetcher.path("/stations").method("post").create()(arg as any, init)

Instead of this:

export const CreateStation2 = fetcher.path("/stations").method("post").create()
// const CreateStation2: (arg: Record<string, never>, init?: RequestInit | undefined) => ...

This PR fixes this by changing the inference of OpArgType slightly. This allows for these types of POST payloads to be sent.
I have also added a test for a slice of my problematic API.

[fedorg]

Since the merge request has stalled, I have published my version on NPM: @fedorg/openapi-typescript-fetch
You can use if you have the same issue with Record<string, never> showing up in arguments. Use at least version ^1.1.3

[ajaishankar]

HI @fedorg there are a couple of commits in master that has not yet been released (my bad) and a fix addressing this exact issue is one of them b62ee71. Will release the pending changes today, sorry about the confusion and your time...

[duncanbeevers]

These changes were released in 1.1.3. I think this PR should be closed.