Work on challenge 17.

src/bytes.rs

@@ -101,7 +101,7 @@ impl Bytes {
         }
     }
 
-    pub fn has_valid_pkcs7(&mut self, block_size: usize) -> bool {
+    pub fn has_valid_pkcs7(&self, block_size: usize) -> bool {
         if self.len() > 0 && self.len() % block_size != 0 {
             return false;
         }