fix(conversations): unnecessary String allocation in normalize_message_item

`handlers.rs:531` allocates a `String` on every call to
`normalize_message_item` for a type comparison:

```rust
map.get("type") != Some(&Value::String("message".to_owned()))
```

**Fix:**

```rust
map.get("type").and_then(Value::as_str) != Some("message")
```