Fix key error while delete not return content-type

Signed-off-by: Xuanwo <xuanwo@yunify.com>

qingstor/sdk/unpack.py

@@ -51,7 +51,7 @@ def unpack_response_body(self):
         # In other situations, body should not be unpacked for possibly large memory usage
         if not (
                 self.res.ok and
-                self.res.headers["Content-Type"] != "application/json"
+                self.res.headers.get("Content-Type")  != "application/json"
         ):
             try:
                 data = self.res.json()