Fix state pension top up age bug

- Adjusted calculator to cover error which allowed people with their date of birth after the top up end date to be shown a rate.
- Fixed tests.

Relates to:
https://www.pivotaltracker.com/story/show/72886346
```if age > UPPER_AGE || (year == TOPUP_END_DATE.year && birthday_after_topup_end(dob))```

```def birthday_after_topup_end(dob)
     birthday = Date.new(dob.day, dob.month, TOPUP_END_DATE.year)
     birthday > TOPUP_END_DATE
  end```

> or >= to be checked :-(