swift delete_after metadata should be type str not int

[Jake Yip]

When deleting using the `delete_after` method [1], an error is thrown

 requests.exceptions.InvalidHeader: Header part (60) from {'x-delete-after': 60} must be of type str or bytes, not <class 'int'>

This can be replicated by doing the following:

1. Assume you have a `temp` container with a `trash` object

 $ openstack object list temp
 +-----------+
 | Name |
 +-----------+
 | trash |
 +-----------+

2. Delete using the following code

 import openstack
 conn = openstack.connect(cloud='testing')
 sess = conn.session

 obj = conn.object_store.get_object_metadata(container='temp', obj='trash')
 obj.delete_after = 60
 obj = conn.object_store.set_object_metadata(obj)

3. This throws an error with the following stacktrace

Traceback (most recent call last):
  File "/home/jake/.virtualenvs/osc/lib/python3.10/site-packages/keystoneauth1/session.py", line 1022, in _send_request
    resp = self.session.request(method, url, **kwargs)
  File "/home/jake/.virtualenvs/osc/lib/python3.10/site-packages/requests/sessions.py", line 573, in request
    prep = self.prepare_request(req)
  File "/home/jake/.virtualenvs/osc/lib/python3.10/site-packages/requests/sessions.py", line 484, in prepare_request
    p.prepare(
  File "/home/jake/.virtualenvs/osc/lib/python3.10/site-packages/requests/models.py", line 369, in prepare
    self.prepare_headers(headers)
  File "/home/jake/.virtualenvs/osc/lib/python3.10/site-packages/requests/models.py", line 491, in prepare_headers
    check_header_validity(header)
  File "/home/jake/.virtualenvs/osc/lib/python3.10/site-packages/requests/utils.py", line 1037, in check_header_validity
    raise InvalidHeader(
requests.exceptions.InvalidHeader: Header part (60) from {'x-delete-after': 60} must be of type str or bytes, not <class 'int'>

[1] https://docs.openstack.org/openstacksdk/latest/user/resources/object_store/v1/obj.html#openstack.object_store.v1.obj.Object.delete_after

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/openstacksdk/+/924301