Implement IntoBuf for mut slices. (#214)

With this if foo is a mutable slice, it is possible to do

foo.into_buf().put_u32_le(42);

Before this patch into_buf would create a Cursor<&'a [u8]> and it
would not be possible to write into it.

src/buf/into_buf.rs

@@ -63,6 +63,14 @@ impl<'a> IntoBuf for &'a [u8] {
     }
 }
 
+impl<'a> IntoBuf for &'a mut [u8] {
+    type Buf = io::Cursor<&'a mut [u8]>;
+
+    fn into_buf(self) -> Self::Buf {
+        io::Cursor::new(self)
+    }
+}
+
 impl<'a> IntoBuf for &'a str {
     type Buf = io::Cursor<&'a [u8]>;
 

tests/test_bytes.rs

 extern crate bytes;
 
-use bytes::{Bytes, BytesMut, BufMut};
+use bytes::{Bytes, BytesMut, BufMut, IntoBuf};
 
 const LONG: &'static [u8] = b"mary had a little lamb, little lamb, little lamb";
 const SHORT: &'static [u8] = b"hello world";
@@ -303,6 +303,13 @@ fn fns_defined_for_bytes_mut() {
     assert_eq!(&v[..], bytes);
 }
 
+#[test]
+fn mut_into_buf() {
+    let mut v = vec![0, 0, 0, 0];
+    let s = &mut v[..];
+    s.into_buf().put_u32_le(42);
+}
+
 #[test]
 fn reserve_convert() {
     // Inline -> Vec