Address issue #249 and #250 - achieve consistent payoff amount

[wisdant]

Changes included in this PR:

1. Changed payoff amount calculation so that the user pays exactly the same amount to payoff no matter the amount is used when calling makePayment(). (Issue Actual payoff amount can be different when calling makePayment() with different amounts #249
2. Changed the behavior related to default. Do not distribute income when setting an account to Defaulted. Do not distribute income for pay periods afterwards as well even if updateDueInfo() is called. The writeoff amount only includes principal at the moment of default. All the payments from the user are applied towards defaulted principals first. Only after the defaulted principals are all paid, the payment will be applied towards the fees&Interest accrued since the moment of default. The account stays in Defaulted state until everything is paid off. (Post-default payment does not work if the payment happened multiple pay periods after the default event #250 )

[PlayJok3r]

Summary of key changes:

1. Do not updateDueInfo if already in default state.
2. Handles payment differently for default borrowers.