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;

src/buf/ext/mod.rs

@@ -124,6 +124,32 @@ pub trait BufMutExt: BufMut {
     fn writer(self) -> Writer<Self> where Self: Sized {
         writer::new(self)
     }
+
+    /// Creates an adapter which will chain this buffer with another.
+    ///
+    /// The returned `BufMut` instance will first write to all bytes from
+    /// `self`. Afterwards, it will write to `next`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::{BufMut, buf::BufMutExt};
+    ///
+    /// let mut a = [0u8; 5];
+    /// let mut b = [0u8; 6];
+    ///
+    /// let mut chain = (&mut a[..]).chain_mut(&mut b[..]);
+    ///
+    /// chain.put_slice(b"hello world");
+    ///
+    /// assert_eq!(&a[..], b"hello");
+    /// assert_eq!(&b[..], b" world");
+    /// ```
+    fn chain_mut<U: BufMut>(self, next: U) -> Chain<Self, U>
+        where Self: Sized
+    {
+        Chain::new(self, next)
+    }
 }
 
 impl<B: BufMut + ?Sized> BufMutExt for B {}