`assert_equal`: fix `clippy::default_numeric_fallback` #914

Job	Run time
msrv	45s
doc	24s
check format	10s
check	16s
semver-checks	16s
check (--no-default-features)	14s
test	1m 3s
check (--no-default-features --features use_alloc)	19s
check (--all-targets --all-features)	25s
All checks succeeded	1s
	3m 53s

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