Ensure a config is always returned from CsvShaper::Shaper.config

If the config was not previously set up, `CsvShaper::Shaper.config` would return `nil` and break the header inflector's expectation that a config was always present. This ensures that a config object is returned if no config has been set already.

This fixes a regression introduced in v1.1.0.
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.