HTML Entities are not Decoded in Attributes.

```rust
#[test]
fn test() {
lol_html::rewrite_str(
r#"

"#,
lol_html::RewriteStrSettings {
element_content_handlers: vec![
lol_html::element!("img", |e| {
assert_eq!(e.get_attribute("alt").as_deref(), Some("Foo & Bar"));
Ok(())
}),
],
..Default::default()
}).unwrap();
}
```

```rust
thread 'test' panicked at 'assertion failed: `(left == right)`
left: `Some("Foo & Bar")`,
right: `Some("Foo & Bar")`', src/main.rs:10:21
```

Expected behaviour: The entities are decoded.
Actual behaviour: The source representation of the attribute contents are returned.

Workaround: Manually decode the string returned from `get_attribute`.

Start by reproducing the failing Rust test in the issue and trace the get_attribute path used by the img element handler. Done means the handler receives "Foo & Bar" instead of the source text "Foo & Bar", while the existing rewrite operation still succeeds.

Written by the indexing model from the issue text.