#161 Fix Decimal instances deemed plain objects

decimal.js

@@ -4862,7 +4862,7 @@
 
   // Create and configure initial Decimal constructor.
   Decimal = clone(DEFAULTS);
-
+  Decimal.prototype.constructor = Decimal;
   Decimal['default'] = Decimal.Decimal = Decimal;
 
   // Create the internal constants from their string values.

decimal.mjs

@@ -4861,6 +4861,7 @@ P[Symbol.toStringTag] = 'Decimal';
 
 // Create and configure initial Decimal constructor.
 export var Decimal = clone(DEFAULTS);
+Decimal.prototype.constructor = Decimal;
 
 // Create the internal constants from their string values.
 LN10 = new Decimal(LN10);