fix(pluck): fix pluck's catch-all signature for better type safety

[Lonelind]

Description: Change pluck's catch-all signature that is causing troubles when the
passed string is not a key in the argument object.

Related issue (if exists): #5188

[Lonelind]

Just happened to see that there is a line that was added 7 months ago that does virtually the same but with one core difference: if you use R there, it will fail the same way as it was already but it will happen with the seventh argument passed. The pluck operator will resolve type by itself returning what was specified in result (it will fall down to unknown only if it was any there). Passing unknown every time is safer. You will need to check the type, yes but it will not say that everything is okay even if it isn't.

[Lonelind]

Also, I've added a test case that checks type inference:

it('should not infer type from the variable if key doesn\'t exist', () => {
  const a: Observable<number> = of({ name: 'abc' }).pipe(pluck('xyz')); // $ExpectError
});

With the old implementation there's no error emitting and the pluck signature gets number as the R. This fix will return unknown there, and the types don't match since, that is an expected behaviour.

[cartant]

LGTM. Thanks.

[benlesh]

This can't go into patch, because it will break people's builds if they're violating the new constraint. However, it's a good addition to master.