fix(deco): get content type header from request attr

Author: mattwang44

data_spec_validator/decorator/decorators.py

@@ -104,8 +104,7 @@ def _collect_data(method, req_qp, req_data) -> Dict:
             return req_data
 
     def _get_dj_payload(request):
-        content_type = request.headers.get('Content-Type')
-        if 'application/json' in content_type:
+        if 'application/json' in request.content_type:
             try:
                 return request.body and json.loads(request.body) or {}
             except Exception: