Added support to change the default `Content-Type` for json

APIs, such as [JSON API](http://jsonapi.org/format) which expects `Content-Type: application/vnd.api+json`, would be enhanced by establishing the default `Content-Type` outside the `write()` method.

```
class JSONHandler(RequestHandler):
  _json_content_type = "application/vnd.api+json"
  def get(self):
    # ...
    self.finish(json_api_doc)

```