Add inline attribute to BytesMut::as_mut (#410)

src/bytes_mut.rs

@@ -1026,6 +1026,7 @@ impl Deref for BytesMut {
 }
 
 impl AsMut<[u8]> for BytesMut {
+    #[inline]
     fn as_mut(&mut self) -> &mut [u8] {
         self.as_slice_mut()
     }