Proper defaults

Oops, there's a built-in way to specify defaults, let's use that and
drop the silly _UNSPECIFIED member.

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Fix up errors strings

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Adhere to strict encoding-mode requirement in test

Also expose a setter for the encoding-mode() option, to be consistent with
the encoding() option's implementation.

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Add test for encoding-mode(strict) w/o encoding

Signed-off-by: Anton Lofgren <alofgren@op5.com>