Simplify other/extension handling.

[coveralls]

Pull Request Test Coverage Report for Build 5796226902

• 93 of 99 (93.94%) changed or added relevant lines in 10 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.3%) to 93.639%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-types/src/extensions/quota.rs	13	14	92.86%
imap-types/src/flag.rs	21	26	80.77%
<!--	Total:	93	99	93.94%	-->

Files with Coverage Reduction	New Missed Lines	%
imap-types/src/auth.rs	1	84.38%
imap-types/src/flag.rs	1	73.85%
imap-types/src/extensions/enable.rs	2	91.07%
<!--	Total:	4		-->

Totals
Change from base Build 5763210682:	0.3%
Covered Lines:	8714
Relevant Lines:	9306

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💛 - Coveralls