Fix datetime stringification with column names that need escaping #865

Job	Run time
build-vegafusion-python-embed-linux-arm64	3m 4s
build-vegafusion-python-embed-linux-64	2m 45s
build-vegafusion-python-embed-osx-64	5m 51s
build-vegafusion-python-embed-osx-arm64	6m 2s
build-vegafusion-wasm	2m 59s
build-vegafusion-python-embed-win-64	5m 46s
check-rust	5m 43s
test-rust (ubuntu-20.04)	22m 2s
build-vegafusion-jupyter-packages	5m 15s
build-vegafusion-packages	52s
test-rust (windows-2022)	35m 46s
test-rust (macos-11)	53m 23s
build-vegafusion-server-linux-64	2m 8s
build-vegafusion-server-linux-arm64	2m 0s
build-vegafusion-server-osx-64	5m 29s
build-vegafusion-server-osx-arm64	9m 6s
build-vegafusion-server-win-64	4m 50s
test-vegafusion-python-linux-64	6m 53s
test-vegafusion-jupyter-linux-64	24m 12s
test-vegafusion-python-win-64	9m 33s
test-vegafusion-python-osx-64	12m 24s
	3h 46m 3s

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