UnpackArgs has different behavior for `args` vs `kwargs`

[nicks]

Repro steps:

1. Define a built-in function with UnpackArgs used like this:

if err := starlark.UnpackArgs("unpack", args, kwargs,
    "x", &x, "y?", &y, "z", &z); err != nil {
  t.Errorf("UnpackArgs failed: %v", err)
}

2. Pass two arguments to this function as named args:

f(1, 2)

This is OK.

3. Pass two arguments to this function as positional args:

f(x=1, y=2)

Expected result:

This should succeed. y is optional, so z should also be optional

Actual result:

UnpackArgs failed: unpack: missing argument for z

Additional info

I have a test in a branch that demonstrates the problem, might also open a PR if the fix is easy.

[adonovan]

Did you mean starlark.UnpackArgs("unpack", args, kwargs, ...?

[nicks]

oops, yes, fixed!