ts-jest: regression in 0.5.1

[rbnayax]

version 0.5.1 introduced a regression that was a result of #752

Mocked implementation that returns bullish values stopped working.

The following test illustrates the issue:

import { createMock } from '@golevelup/ts-jest';

describe(`test`, () => {
  it('mock called with another mock', async () => {
    interface Test {
      foo(): number | undefined;
    }

    const mock = createMock<Test>();
    mock.foo.mockImplementation(() => {
      return 6;
    });
    expect(mock.foo()).toEqual(6);
    mock.foo.mockImplementation(() => {
      return 7;
    });
    expect(mock.foo()).toEqual(7);
    mock.foo.mockImplementation(() => {
      return 0;
    });
    expect(mock.foo()).toEqual(0); // Fails
    mock.foo.mockImplementation(() => {
      return undefined;
    });
    expect(mock.foo()).toEqual(undefined); //Fails
  });
});

Error: Unexpected return from a matcher function.
Matcher functions should return an object in the following format:
  {message?: string | function, pass: boolean}
'{"actual": [Function mockConstructor], "expected": 0, "message": [Function anonymous], "name": "toEqual", "pass": [Function mockConstructor]}' was returned