JsonResult to derive from ObjectResult.

should we initialize `ContentTypes` in the constructor?

Thinking about the scenario for hypermedia where I want to add support for a user-defined-json-encoded-content-type as well as the defaults.

```
var json = new JsonResult(new { foo = "bar"});
json.ContentTypes.Add("application/my-thing+json");
```

**VS**

```
var json = new JsonResult(new { foo = "bar"});
json.ContentTypes.Add("application/my-thing+json");
json.ContentTypes.Add("application/json");
json.ContentTypes.Add("text/json");
```