td.explain should indicate thenCallback / thenResolve / thenReject from thenReturn

[moeriki]

var fn = td.func();
td.when(fn()).thenResolve(true);
console.log(td.explain(fn));

Expected

{
  callCount: 0,
  calls: [],
  description: 'This test double has 1 stubbings and 0 invocations.\n\nStubbings:\n  - when called with `()`, then resolve `null`.',
  isTestDouble: true,
}

The important part is then resolve 'null'.

Actual

{
  callCount: 0,
  calls: [],
  description: 'This test double has 1 stubbings and 0 invocations.\n\nStubbings:\n  - when called with `()`, then return `null`.',
  isTestDouble: true,
}

I can make a pull request if accepted.