Fix borrowed Cow<'_, [u8]> deserializing as str.

This changes the deserialize implementation for a borrowed Cow<[u8]>
to specifically request a byte slice, rather than a borrowed string.

The old behavior breaks any program which relies on data being
deserialized the same way as it was serialized and uses Cow<[u8]>.
In serde_json, it just wouldn't deserialize. In bincode, it
deserialized normally unless the bytes were invalid UTF8.

Fixes https://github.com/TyOverby/bincode/issues/231.

serde/src/private/de.rs

@@ -188,7 +188,7 @@ where
         }
     }
 
-    deserializer.deserialize_str(CowBytesVisitor)
+    deserializer.deserialize_bytes(CowBytesVisitor)
 }
 
 pub mod size_hint {