text: Title: [From flask-restplus #589]  Stop using failing ujson as default serializer
  Body: Flask-restx uses `ujson` for serialization by default, falling back to normal `json` if unavailable.
  As has been mentioned before `ujson` is quite flawed, with some people reporting rounding errors.
  
  Additionally, `ujson.dumps` does not take the `default` argument, which allows one to pass any custom method as the serializer. Since my project uses some custom serialization, I have been forced to fork flask-restx and just remove the ujson import line... 
  
  You could add an option to choose which lib to use, or completely remove ujson from there. I think it is bad practice to have to maintain the code for two lbraries with different APIs. 
  An alternative could be to make ujson an optional dependency and not list it in the package dependencies, but keep the current imports as they are. As a result, anyone wanting to use ujson can install it and it will take over transparently.
  
  Original issue: `https://github.com/noirbizarre/flask-restplus/issues/589`
  
  I would gladly make the PR if you are happy with one of these solutions