Added response to whenPOST mock

**Request Type**: docs

**How to reproduce:** Use whenPOST without returning a response

**Component(s):** ngMocks

**Impact:** small

**Complexity:** small

**This issue is related to:** whenPOST (mocking)

**Detailed Description:** The  ```whenPOST``` method should return a response containing status, response body and headers. If omitted the following error will be thrown:

```Uncaught TypeError: Cannot read property '2' of undefined```

The documentation doesn't make it very clear, so I think it will be appropriate to add.