Fix #490: Display appropriate separator when writing default enumerable values

[gfs]

Currently, the default values for an IEnumerable option are always written with spaces separating them, even when the separator is set to something else. This change reads the configured separator from the option to include in the generated Default value for the HelpText.

Adds a new test case to confirm behavior when separator is configured. Previous behavior is maintained if separator is not configured.

Fix #490