`assert_equal`: fix `clippy::default_numeric_fallback` #911

Job	Run time
check format	12s
doc	14s
check	14s
msrv	44s
check (--no-default-features)	12s
check (--no-default-features --features use_alloc)	13s
semver-checks	2m 34s
check (--all-targets --all-features)	24s
test	57s
All checks succeeded	1s
	5m 45s

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