Convert all content types to string #847

Job	Run time
unit-tests (ubuntu-latest, nightly)	37s
unit-tests (macos-latest, v0.11.2)	54s
unit-tests (windows-latest, v0.11.2)	1m 12s
unit-tests (ubuntu-latest, v0.11.2)	1m 17s
unit-tests (windows-latest, nightly)	1m 11s
unit-tests (macos-latest, nightly)	56s
	6m 7s

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