Add quirk for encoding NIL body-fld-enc to empty istring

[soywod]

Fixes #640.

[coveralls]

Pull Request Test Coverage Report for Build 13200444457

Details

• 7 of 8 (87.5%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.003%) to 91.627%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-codec/src/body.rs	7	8	87.5%

Totals
Change from base Build 12994428533:	-0.003%
Covered Lines:	11468
Relevant Lines:	12516

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💛 - Coveralls

[duesee]

Thanks! Looks good to me!