Fix Date's NaN JSON.stringify (#764)

Jint.Tests/Runtime/DateTests.cs

@@ -57,5 +57,11 @@ namespace Jint.Tests.Runtime
             Assert.Equal("Invalid Date", value);
         }
 
+        [Fact]
+        public void ToJsonFromNaNObject()
+        {
+            var result = _engine.Execute("JSON.stringify({ date: new Date(NaN) });").GetCompletionValue();
+            Assert.Equal("{\"date\":null}", result.ToString());
+        }
     }
 }

Jint/Native/Date/DatePrototype.cs

@@ -688,7 +688,7 @@ namespace Jint.Native.Date
         {
             var o = TypeConverter.ToObject(Engine, thisObj);
             var tv = TypeConverter.ToPrimitive(o, Types.Number);
-            if (tv.IsNumber() && double.IsInfinity(((JsNumber) tv)._value))
+            if (tv.IsNumber() && !IsFinite(((JsNumber) tv)._value))
             {
                 return Null;
             }