Fix NotSupportedException when returning IEnumerable<ContentBlock> #615

Job	Run time
build (ubuntu-latest, Release)	3m 2s
build (macos-latest, Debug)	4m 22s
build (windows-latest, Debug)	4m 48s
build (windows-latest, Release)	4m 49s
build (macos-latest, Release)	5m 33s
build (ubuntu-latest, Debug)	3m 31s
publish-coverage	0s
	26m 5s

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