scylla_util.py: change default value for headers argument in curl function
scylla_util.py: change default value for headers argument in curl function

- It was set to {} that is incorrect and can lead to unexpected behavior
https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments
- Order of the arguments changed to more convinient way

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla