Fix got.mergeOptions(...) types

[scttcper]

Adds test coverage for mergeOptions usage similar to that in the readme.

Not passing because I'm not sure what the fix should be, willing to make the change. Partial?

[szmarczak]

The fix is to replace

got/source/create.ts

Line 73 in d11dbe4

mergeOptions<T extends PartialDeep<Options>>(...sources: T[]): T;

with

	mergeOptions(...sources: Options[]): NormalizedOptions;

I forgot to update it in #921

I'm not sure how to test the types though. Should we use tsd?

/cc @sindresorhus

[sindresorhus]

I'm not sure how to test the types though. Should we use tsd?

tsd is meant to test TypeScript definition files for JS projects.

If we really need to test the actual types, we could use something like this:

type AssertEqual<T, Expected> =
  T extends Expected
  ? (Expected extends T ? true : never)
  : never;

But I don't think it's needed in this case.