make method instead of property for alternative json decoding of request

sanic/request.py

@@ -69,8 +69,18 @@ class Request(dict):
         self.stream = None
 
     @property
-    def json(self, loads=json_loads):
+    def json(self):
         if self.parsed_json is None:
+            try:
+                self.parsed_json = json_loads(self.body)
+            except Exception:
+                if not self.body:
+                    return None
+                raise InvalidUsage("Failed when parsing body as json")
+
+        return self.parsed_json
+
+    def load_json(self, loads=json_loads):
         try:
             self.parsed_json = loads(self.body)
         except Exception: