Set content-type to application/json by default.

When making calls using module `robottelo.api.client`, ensure that those calls have a default content-type of application/json. If the user has already set a content-type, do not override that setting. If the content-type of a request is application/json, automatically encode the request data. This change necessitates several other changes:

* Create private helper functions in module `robottelo.api.client`. Add test functions to module `tests.robottelo.test_robottelo_api_client`.
* Update the module-wide docstring in module `robottelo.api.client` to describe the new behaviour of the API client.
* Trim now-useless information out of module `robottelo.entities`.

Refactor existing tests. Rather than iterating through sets of data within a test function, generate multiple test functions using the ddt module.

Refactor existing docstrings by improving intra-documentation linking.