[Tests] Send only string headers to align to new requests limitation

[Hedingber]

We started seeing those failures in CI:

E               requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.restricted: 'restricted'>) from {'x-mlrun-deletion-strategy': <DeletionStrategy.restricted: 'restricted'>} must be of type str or bytes, not <enum 'DeletionStrategy'>

/usr/local/lib/python3.7/site-packages/requests/utils.py:1038: InvalidHeader
FAILED tests/api/api/test_projects.py::test_delete_project_with_resources[leader] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.check: 'check'>) from {'x-mlrun-deletion-strategy': <De...
FAILED tests/api/api/test_projects.py::test_delete_project_with_resources[follower] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.check: 'check'>) from {'x-mlrun-deletion-strategy': <...
FAILED tests/api/api/test_projects.py::test_delete_project_deletion_strategy_check[leader] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.check: 'check'>) from {'x-mlrun-deletion-strat...
FAILED tests/api/api/test_projects.py::test_delete_project_deletion_strategy_check[follower] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.check: 'check'>) from {'x-mlrun-deletion-str...
FAILED tests/api/api/test_projects.py::test_delete_project_not_deleting_versioned_objects_multiple_times[leader] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.cascading: 'cascading'>)...
FAILED tests/api/api/test_projects.py::test_delete_project_not_deleting_versioned_objects_multiple_times[follower] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.cascading: 'cascading'...
FAILED tests/api/api/test_projects.py::test_delete_project_deletion_strategy_check_external_resource[leader] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.restricted: 'restricted'>) f...
FAILED tests/api/api/test_projects.py::test_delete_project_deletion_strategy_check_external_resource[follower] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.restricted: 'restricted'>)...
FAILED tests/api/api/test_projects.py::test_projects_crud[leader] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.restricted: 'restricted'>) from {'x-mlrun-deletion-strategy': <Deletion...
FAILED tests/api/api/test_projects.py::test_projects_crud[follower] - requests.exceptions.InvalidHeader: Header part (<DeletionStrategy.restricted: 'restricted'>) from {'x-mlrun-deletion-strategy': <Deleti...

Apparently requests released 2.28.0 in which they started enforcing that headers are only of type string or bytes, therefore had to fix those tests that were sending enum values
In addition I went through our clients that use requests (httpdb, iguazio and nuclio) and added a general handling to enum headers or params