repository tests using apis
Do not check content-type of HTTP 204 messages

Checking the content-type of an HTTP 204 message does not make sense, as HTTP
204 messages "MUST NOT include a message-body". Make the test suite reflect this
fact.
Merge pull request #1274 from sghai/repo-api

repository tests using apis
Merge pull request #1286 from Ichimonji10/http-204

Do not check content-type of HTTP 204 messages
Make ActivationKey readable, trim down tests

Make `ActivationKey` inherit from `EntityReadMixin`. Use this mixin to trim down
module `tests.foreman.api.test_activationkey_v2`: replace many usages of
`client.get` with `ActivationKey.read_json`.

All tests pass or are skipped.