dd: fix ISO-8859-1 case conversion for conv=lcase/ucase #923

Job	Run time
L10n/Multi-call Binary Install Test (macos-latest, feat_os_macos)	6m 33s
L10n/Clap Error Localization Test	2m 23s
L10n/Multi-call Binary Install Test (ubuntu-latest, feat_os_unix)	4m 20s
L10n/Fluent Syntax Check	15s
L10n/Embedded Locale Regression Test	2m 12s
L10n/Installation Test (Make & Cargo) (macos-latest, feat_os_macos)	14m 58s
L10n/Build and Test (ubuntu-latest, feat_os_unix)	4m 1s
L10n/French Integration Test	2m 26s
L10n/Build and Test (macos-latest, feat_os_macos)	4m 10s
L10n/Locale Embedding - Cat Utility	1m 16s
L10n/Locale Embedding - Cargo Install	1m 30s
L10n/Locale Support Verification	8m 32s
L10n/Installation Test (Make & Cargo) (ubuntu-latest, feat_os_unix)	7m 15s
L10n/Locale Embedding - Ls Utility	1m 16s
L10n/Locale Embedding - Multicall Binary	2m 58s
L10n/Build and Test (windows-latest, feat_os_windows)	9m 0s
L10n/Locale Embedding Regression Test	3s
	1h 13m 8s

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