String::fromNumber gave a wrong result with uint8_t and int8_t values #668

Job	Run time
ios-sdl	2m 44s
android-sdl	5m 4s
ios-sfml-graphics	53s
android-sfml-graphics	1m 34s
linux	5m 3s
linux-latest-dev	4m 31s
linux-oldest	58s
linux-per-backend	5m 33s
macos	2m 6s
windows	2m 48s
windows-clang	5m 3s
windows-oldest	6m 23s
windows-static-mt	5m 8s
nightly-build-windows-visual-studio	0s
	47m 48s

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