Convert all content types to string #834

Job	Run time
unit-tests (ubuntu-latest, nightly)	31s
unit-tests (windows-latest, nightly)	1m 8s
unit-tests (macos-latest, nightly)	49s
unit-tests (ubuntu-latest, v0.11.1)	1m 25s
unit-tests (macos-latest, v0.11.1)	50s
unit-tests (windows-latest, v0.11.1)	1m 10s
	5m 53s

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