chunk_capacity() does not decrement as expected

[jgraettinger]

Documentation for chunk_capacity() says:

Gets the remaining capacity in the current chunk (in bytes).

However:

#[test]
fn test_chunk_capacity() {
    let alloc = bumpalo::Bump::with_capacity(1 << 15);
    assert_eq!(alloc.chunk_capacity(), 36800);

    // Allocation which fits within the current chunk.
    alloc.alloc_str("hello world");

    // Expect chunk capacity is lower than before, because of the allocation.
    assert_ne!(alloc.chunk_capacity(), 36800);
}

Actual output as of release 3.11.0 is:

running 1 test
thread 'test_chunk_capacity' panicked at 'assertion failed: `(left != right)`
  left: `36800`,
 right: `36800`', my_test.rs

[msullivan]

I think #209 fixed this