Make freeze respect the start offset for BytesMuts in Vec mode

[TimHambourger]

Fixes #352. This one seemed pretty straightforward to me (advance the Bytes before returning). But I'm a Rust newbie, so let me know if I'm missing something.

Also, I guess this is technically a breaking change b/c consuming code might have been depending on the old behavior. E.g. something like

let mut b: BytesMut;
b.advance(2);
let b = b.freeze();
// Huh, weird, frozen version doesn't reflect my advance. Guess I'll advance again.
b.advance(2);

Y'all's call on how big of a concern that is. My thought is it seems small: The work around above seems less likely than the workaround of using split_to as mentioned in the original issue. And the old behavior definitely seemed inconsistent (behavior for Vec mode differed from the behavior for Arc mode, and behavior for freeze differed from the behavior for as_slice/as_slice_mut).

[seanmonstar]

Thanks for fixing this!

I get what you're saying about this being breaking, but that's the reality of any bug that people try to work around...