improve-config

- All `Vault::Config` methods have docstrings.  All methods have tests
  now too.

- A new `Vault::Config.array` method splits environment variables with
  comma-separated strings into string arrays.

- `Vault::Test::EnvironmentHelpers` is no longer included in
  `Vault::TestCase` by default.  If needed it must be manually
  included.

- Add additional tests around SSL enable/disable functionality.