refs https://github.com/TryGhost/Team/issues/1104
- `.get('key')` would error because it was incorrectly expecting the internal content object to contain `{key, value}` objects
- changed `.getAll()` to return a shallow copy of the internal content object so we don't leak references that can result in unexpected changes to the cache content
- changed the internal content object naming to `_content` to highlight that it's an internal/private implementation detail rather than a public API
