ujson segfaults with numpy arrays

[jackieleng]

In flask_restplus.representations:output_json (https://github.com/noirbizarre/flask-restplus/blob/master/flask_restplus/representations.py#L25)
usjon.dumps is called (when installed) with indent=4 when DEBUG is enabled. However this is causing a segfault when you're trying to serialize a numpy array. Without indent it also causes an error, but it's an OverflowError. See for example this issue: ultrajson/ultrajson#294

I know it's a problem with ujson, but it does kill the server and it's not easy to find the cause.

[ziirish]

I think we will probably drop ujson as stated in #589 because the project does not seem actively supported anymore.

Thanks for the report.