minor test issue: in tests/slab.rs reserver_exact() should be called and instead reserve() is called.

[BruceBrown]

This looks like a cut-and-paste thing. Around line 190, there's a reserve() call in a test that seems to want to test reserve_exact()

#[test]
fn reserve_exact_does_not_allocate_if_available() {
...
slab.reserve(8);
assert_eq!(10, slab.capacity());
}