Fixes compatibility with Werkzeug 2.1.0 ported over from flask-restx#423.

https://github.com/python-restx/flask-restx/pull/423

diff --git a/flask_restful/reqparse.py b/flask_restful/reqparse.py
index 9bb3099..5c59594 100644
--- a/flask_restful/reqparse.py
+++ b/flask_restful/reqparse.py
@@ -114,7 +114,10 @@ class Argument(object):
         :param request: The flask request object to parse arguments from
         """
         if isinstance(self.location, six.string_types):
-            value = getattr(request, self.location, MultiDict())
+            if self.location in {"json", "get_json"}:
+                value = request.get_json(silent=True)
+            else:
+                value = getattr(request, self.location, MultiDict())
             if callable(value):
                 value = value()
             if value is not None:
@@ -122,7 +125,10 @@ class Argument(object):
         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:
diff --git a/tests/test_api.py b/tests/test_api.py
index 15f12eb..9a9cceb 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -936,7 +936,7 @@ class APITestCase(unittest.TestCase):
         app = app.test_client()
         resp = app.get('/api')
         self.assertEqual(resp.status_code, 302)
-        self.assertEqual(resp.headers['Location'], 'http://localhost/')
+        self.assertEqual(resp.headers['Location'], '/')
 
     def test_json_float_marshalled(self):
         app = Flask(__name__)
diff --git a/tests/test_reqparse.py b/tests/test_reqparse.py
index 1d75e40..e5c586b 100644
--- a/tests/test_reqparse.py
+++ b/tests/test_reqparse.py
@@ -23,8 +23,9 @@ class ReqParseTestCase(unittest.TestCase):
         with app.app_context():
             parser = RequestParser()
             parser.add_argument('foo', choices=('one', 'two'), help='Bad choice: {error_msg}')
-            req = Mock(['values'])
+            req = Mock(["values", "get_json"])
             req.values = MultiDict([('foo', 'three')])
+            req.get_json.return_value = None
             parser.parse_args(req)
             expected = {'foo': 'Bad choice: three is not a valid choice'}
             abort.assert_called_with(400, message=expected)
@@ -35,8 +36,9 @@ class ReqParseTestCase(unittest.TestCase):
         with app.app_context():
             parser = RequestParser()
             parser.add_argument('foo', choices=('one', 'two'), help=u'Bad choice: {error_msg}')
-            req = Mock(['values'])
+            req = Mock(["values", "get_json"])
             req.values = MultiDict([('foo', u'\xf0\x9f\x8d\x95')])
+            req.get_json.return_value = None
             parser.parse_args(req)
             expected = {'foo': u'Bad choice: \xf0\x9f\x8d\x95 is not a valid choice'}
             abort.assert_called_with(400, message=expected)
@@ -47,8 +49,9 @@ class ReqParseTestCase(unittest.TestCase):
         with app.app_context():
             parser = RequestParser()
             parser.add_argument('foo', choices=['one', 'two'], help='Please select a valid choice')
-            req = Mock(['values'])
+            req = Mock(["values", "get_json"])
             req.values = MultiDict([('foo', 'three')])
+            req.get_json.return_value = None
             parser.parse_args(req)
             expected = {'foo': 'Please select a valid choice'}
             abort.assert_called_with(400, message=expected)
@@ -58,8 +61,9 @@ class ReqParseTestCase(unittest.TestCase):
         def bad_choice():
             parser = RequestParser()
             parser.add_argument('foo', choices=['one', 'two'])
-            req = Mock(['values'])
+            req = Mock(["values", "get_json"])
             req.values = MultiDict([('foo', 'three')])
+            req.get_json.return_value = None
             parser.parse_args(req)
             abort.assert_called_with(400, message='three is not a valid choice')
         app = Flask(__name__)
@@ -190,7 +194,8 @@ class ReqParseTestCase(unittest.TestCase):
         self.assertTrue(len(arg.source(req)) == 0)  # yes, basically you don't find it
 
     def test_source_default_location(self):
-        req = Mock(['values'])
+        req = Mock(['values', 'get_json'])
+        req.get_json.return_value = None
         req._get_child_mock = lambda **kwargs: MultiDict()
         arg = Argument('foo')
         self.assertEqual(arg.source(req), req.values)
@@ -215,8 +220,9 @@ class ReqParseTestCase(unittest.TestCase):
         args = parser.parse_args(req)
         self.assertEqual(args['foo'], "bar")
 
-        req = Mock()
+        req = Mock(['get_json'])
         req.values = ()
+        req.get_json.return_value = None
         req.json = None
         req.view_args = {"foo": "bar"}
         parser = RequestParser()