Not to consume buf in the decode_string

Decoding error when processing continuation frames

Author: caiyuanhao

src/hpack/decoder.rs

@@ -319,7 +319,9 @@ impl Decoder {
             return ret;
         }
 
-        Ok(take(buf, len))
+        let v = buf.chunk().copy_to_bytes(len);
+        buf.advance(len);
+        Ok(v)
     }
 }