Handle Werkzeug 2.1.0 change to Request.get_json().

pallets/werkzeug#2339 changed the behavior of `Request.get_json()`
and the `Request.json` property to raise a `BadRequest` if `Request.get_json()`
is called without `silent=True`, or the `Request.json` property is accessed,
and the content type is not `"application/json"`.

Argument parsing allows parsing from multiple locations, and defaults to
`["json", "values"]`, but if the locations include `"json"` and the content
type is not `"application/json"`, a `BadRequest` is now raised with Werkzeug >= 2.1.0.

Invoking `Request.get_json()` with the `silent=True` parameter now handles
the situation where `"json"` is included in the locations, but the content type
is not `"application/json"`.

flask_restx/reqparse.py

@@ -146,7 +146,10 @@ def source(self, request):
         else:
             values = MultiDict()
             for l in self.location:
-                value = getattr(request, l, None)
+                if l in {"json", "get_json"}:
+                    value = request.get_json(silent=True)
+                else:
+                    value = getattr(request, l, None)
                 if callable(value):
                     value = value()
                 if value is not None: