chore: added check for unset values (#771)

Author: tiwarishubham635

twilio/base/serialize.py

@@ -62,6 +62,9 @@ def flatten_dict(d, result=None, prv_keys=None):
 
 
 def boolean_to_string(bool_or_str):
+    if bool_or_str == values.unset:
+        return bool_or_str
+
     if bool_or_str is None:
         return bool_or_str