Use requests.header instead of plain dict

moto/core/responses.py

@@ -131,7 +131,7 @@ class BaseResponse(_TemplateEnvironmentMixin):
         else:
             self.region = self.default_region
 
-        self.headers = dict(request.headers)
+        self.headers = request.headers
         self.response_headers = headers
         return self.call_action()
 

moto/kms/responses.py

@@ -39,8 +39,9 @@ class KmsResponse(BaseResponse):
         try:
             key = self.kms_backend.describe_key(key_id)
         except KeyError:
-            self.headers['status'] = 404
-            return "{}", self.headers
+            headers = dict(self.headers)
+            headers['status'] = 404
+            return "{}", headers
         return json.dumps(key.to_dict())
 
     def list_keys(self):