Support for Stub#resolvesThis()

[kanehara]

• Sinon version : 2.1.0
• Environment : Node 7.7.4

What did you expect to happen?
Stubs have #returnsThis() and #resolves() functionality but it is not possible to use either to return the Stub's context as a Promise. It would be nice to have a #resolvesThis() function in Stubs. For example, Mongoose's Model#save() method returns the saved Model object on success. Given the following function:

function save(userInfo) {
  const user = new User({
    ...userInfo
  });
  return user.save().catch(err => console.log(err));  
}

It would be nice to test that the saved user object actually contains all the info in userInfo via:

const stub = sinon.stub(User, "save");
stub.resolvesThis();
const savedUser = await save(mockUserInfo);
assert(savedUser.id === mockUserInfo.id);

#returnsThis returns the context but not as a promise and #resolves does not allow for returning the Stub context.