Interop with ConcatenatedString can throw exception (#630)

Jint.Tests/Runtime/InteropTests.cs

@@ -1814,5 +1814,15 @@ namespace Jint.Tests.Runtime
             ");
         }
 
+        [Fact]
+        public void ShouldBeAbleToPlusAssignStringProperty()
+        {
+            var p = new Person();
+            var engine = new Engine();
+            engine.SetValue("P", p);
+            engine.Execute("P.Name = 'b';");
+            engine.Execute("P.Name += 'c';");
+            Assert.Equal("bc", p.Name);
+        } 
     }
 }

Jint/Native/JsString.cs

@@ -200,10 +200,7 @@ namespace Jint.Native
 
             public override int Length => _stringBuilder?.Length ?? _value?.Length ?? 0;
 
-            public override object ToObject()
-            {
-                return _stringBuilder.ToString();
-            }
+            public override object ToObject() => ToString();
 
             public override bool Equals(JsValue other)
             {