use dependency injection to allow alternative json parser or encoder

sanic/request.py

@@ -69,10 +69,10 @@ class Request(dict):
         self.stream = None
 
     @property
-    def json(self):
+    def json(self, loads=json_loads):
         if self.parsed_json is None:
             try:
-                self.parsed_json = json_loads(self.body)
+                self.parsed_json = loads(self.body)
             except Exception:
                 if not self.body:
                     return None

sanic/response.py

@@ -237,7 +237,8 @@ class HTTPResponse(BaseHTTPResponse):
 
 
 def json(body, status=200, headers=None,
-         content_type="application/json", **kwargs):
+         content_type="application/json", dumps=json_dumps,
+         **kwargs):
     """
     Returns response object with body in json format.
 
@@ -246,7 +247,7 @@ def json(body, status=200, headers=None,
     :param headers: Custom Headers.
     :param kwargs: Remaining arguments that are passed to the json encoder.
     """
-    return HTTPResponse(json_dumps(body, **kwargs), headers=headers,
+    return HTTPResponse(dumps(body, **kwargs), headers=headers,
                         status=status, content_type=content_type)