Round up to power of 2 is not necessary, because `reserve` already
doubles previous capacity in

```
	new_cap = cmp::max(
		cmp::max(v.capacity() << 1, new_cap),
		original_capacity);
```

which makes `reserve` calls constant in average. Avoiding rounding
up prevents `reserve` from wasting space when caller knows exactly
what space they need.

Patch adds three tests which would fail before this test. The most
important is this:

```
#[test]
fn reserve_in_arc_unique_does_not_overallocate() {
    let mut bytes = BytesMut::with_capacity(1000);
    bytes.take();

    // now bytes is Arc and refcount == 1

    assert_eq!(1000, bytes.capacity());
    bytes.reserve(2001);
    assert_eq!(2001, bytes.capacity());
}
```

It asserts that when user requests more than double of current
capacity, exactly the requested amount of memory is allocated and
is not wasted to next power of two.