Connection: add unit tests for Client methods.

#### Is your feature request related to a problem? Please describe.
* No.

#### Describe the solution you'd like
* We don't have unit tests for some of the Client methods. Add unit tests for the `wpcom_json_api_request_as_user`, `wpcom_json_api_request_as_blog`, and `validate_args_for_wpcom_json_api_request` methods.

#### Describe alternatives you've considered
* n/a

#### Additional context
* n/a

Locate the Connection Client implementation and inspect the wpcom_json_api_request_as_user, wpcom_json_api_request_as_blog, and validate_args_for_wpcom_json_api_request methods. Find the existing unit-test structure for this client, then add coverage for the named methods; done means each method and its argument validation behavior is tested.

Written by the indexing model from the issue text.