Various code cleanup.

app/Models/CurrencyExchangeRate.php

@@ -43,8 +43,16 @@ use Illuminate\Database\Eloquent\Relations\BelongsTo;
  */
 class CurrencyExchangeRate extends Model
 {
-    /** @var array */
-    protected $dates = ['date'];
+    /** @var array Convert these fields to other data types */
+    protected $casts
+        = [
+            'created_at' => 'datetime',
+            'updated_at' => 'datetime',
+            'user_id'          => 'int',
+            'from_currency_id' => 'int',
+            'to_currency_id'   => 'int',
+            'date'             => 'datetime',
+        ];
 
     /**
      * @codeCoverageIgnore