loan: Set the first valuation as the original principal. (#2088)

Fix: #1645.

app/controllers/loans_controller.rb

@@ -2,6 +2,6 @@ class LoansController < ApplicationController
   include AccountableResource
 
   permitted_accountable_attributes(
-    :id, :rate_type, :interest_rate, :term_months
+    :id, :rate_type, :interest_rate, :term_months, :initial_balance
   )
 end