Case now checks :string instead of defaulting

[nha]

Following the discussion on slack.

The only keyword values allowed are :string and :inherit.
:inherit is explicitly not allowed when calling copy so it seems safe to constrain it

process/src/babashka/process.clj

Lines 229 to 234 in 4c6699d

	out (if (and out (not (identical? :inherit out)))
	(future (copy stdout out out-enc))
	stdout)
	err (if (and err (not (identical? :inherit err)))
	(future (copy stderr err err-enc))
	stderr)]