Support Pulp 2.7+ in user-related test

[Ichimonji10]

Method test_use_deleted_user in class pulp_smash.tests.platform.api_v2.test_user.ReadUpdateDeleteTestCase isn't compatible with Pulp 2.7 and beyond. As of Pulp 2.7, calls to endpoints that require JSON but do not have a request body fail with a 400. This test fails because the PUT request requires a request body.

This should be addressed after #29.

(Thanks @asmacdo!)