Convert all content types to string #825

Job	Run time
unit-tests (windows-latest, nightly)	1m 10s
unit-tests (ubuntu-latest, v0.11.1)	34s
unit-tests (macos-latest, nightly)	1m 9s
unit-tests (macos-latest, v0.11.1)	1m 1s
unit-tests (windows-latest, v0.11.1)	1m 19s
unit-tests (ubuntu-latest, nightly)	37s
	5m 50s

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