implicitly grow BytesMut; add BufMutExt::chain_mut (#316)

This brings `BytesMut` in line with `Vec<u8>` behavior.

This also fixes an existing bug in BytesMut::bytes_mut that exposes
invalid slices. The bug was recently introduced and was only on master
and never released to `crates.io`.

In order to fix a test, `BufMutExt::chain_mut` is provided. Withou this,
it is not possible to chain two `&mut [u8]`.

Closes #170

src/buf/buf_mut.rs

@@ -270,7 +270,7 @@ pub trait BufMut {
     fn put_slice(&mut self, src: &[u8]) {
         let mut off = 0;
 
-        assert!(self.remaining_mut() >= src.len(), "buffer overflow");
+        assert!(self.remaining_mut() >= src.len(), "buffer overflow; remaining = {}; src = {}", self.remaining_mut(), src.len());
 
         while off < src.len() {
             let cnt;