value_source_location() returns a bogus span for valueless attributes, and whether it does depends on chunk size

let mut rewriter = HtmlRewriter::new(
Settings::new().append_element_content_handler(element!("div", move |el| {
sink.borrow_mut().extend(el.attributes().iter().map(|a| {
(
a.name_source_location().map(|l| l.bytes()),
a.value_source_location().map(|l| l.bytes()),
)
}));
Ok(())
})),
|_: &[u8]| {},
);

for part in html.chunks(chunk) {
rewriter.write(part).unwrap();
}
rewriter.end().unwrap();

let out = collected.borrow().clone();
out
}

/// Source locations describe the input, so they cannot depend on how the
/// caller happened to split it.
#[test]
fn attribute_spans_do_not_depend_on_chunk_size() {
let whole = spans(PAD_THEN_BARE, PAD_THEN_BARE.len());
for chunk in [1, 4, 8, 16] {
assert_eq!(
spans(PAD_THEN_BARE, chunk),
whole,
"spans changed at chunk size {chunk}"
);
}
}

/// A value is written after its own name, so a value span starting before the
/// name ends is not pointing at a value.
#[test]
fn a_value_span_never_precedes_its_own_name() {
for chunk in [1, 4, 8, 16, PAD_THEN_BARE.len()] {
for (name, value) in spans(PAD_THEN_BARE, chunk) {
if let (Some(name), Some(value)) = (name, value) {
assert!(
value.start >= name.end,
"chunk size {chunk}: value span {value:?} starts before the end \
of its own name span {name:?}"
);
}
}
}
}

/// Control. An attribute that does have a value is already correct and already
/// chunk-independent, so the two tests above are about the valueless case only.
#[test]
fn a_valued_attribute_is_correct_and_chunk_independent() {
let whole = spans(PAD_THEN_VALUED, PAD_THEN_VALUED.len());
assert_eq!(whole.len(), 1);

let (name, value) = whole[0].clone();
assert_eq!(&PAD_THEN_VALUED[name.unwrap()], b"id");
assert_eq!(&PAD_THEN_VALUED[value.unwrap()], b"x");

for chunk in [1, 4, 8, 16] {
assert_eq!(
spans(PAD_THEN_VALUED, chunk),
whole,
"spans changed at chunk size {chunk}"
);
}
}
```

```
test a_valued_attribute_is_correct_and_chunk_independent ... ok
test attribute_spans_do_not_depend_on_chunk_size ... FAILED
test a_value_span_never_precedes_its_own_name ... FAILED

---- attribute_spans_do_not_depend_on_chunk_size ----
assertion `left == right` failed: spans changed at chunk size 1
left: [(Some(15..21), Some(10..10))]
right: [(None, None)]

---- a_value_span_never_precedes_its_own_name ----
chunk size 1: value span 10..10 starts before the end of its own name span 15..21
```

Byte 10 is where `