https://fetch.spec.whatwg.org/#concept-response-clone
If the header guard of the response to clone is `immutable`, then copying the headers to the new response will fail with `Guard is immutable` unless we ensure the guard is copied _after_ the headers.
8650794391/components/script/dom/response.rs (L331-L334)
fn Append(&self, name: ByteString, value: ByteString) -> ErrorResult {
// Step 1
let value = normalize_value(value);
// Step 2
let (mut valid_name, valid_value) = validate_name_and_value(name, value)?;
valid_name = valid_name.to_lowercase();
// Step 3
if self.guard.get() == Guard::Immutable {
return Err(Error::Type("Guard is immutable".to_string()));
}
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