concept-response-clone: Ensure header guard is cloned after headers

[negator]

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.

servo/components/script/dom/response.rs

Lines 331 to 334 in 8650794

	// Step 2
	let new_response = Response::new(&self.global());
	new_response.Headers().set_guard(self.Headers().get_guard());
	new_response.Headers().copy_from_headers(self.Headers())?;

servo/components/script/dom/headers.rs

Lines 202 to 210 in 8650794

	pub fn copy_from_headers(&self, headers: DomRoot<Headers>) -> ErrorResult {
	for (name, value) in headers.header_list.borrow().iter() {
	self.Append(
	ByteString::new(Vec::from(name.as_str())),
	ByteString::new(Vec::from(value.as_bytes())),
	)?;
	}
	Ok(())
	}

servo/components/script/dom/headers.rs

Line 73 in 8650794

return Err(Error::Type("Guard is immutable".to_string()));

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #___ (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because ___

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/dom/response.rs
• @KiChjang: components/script/dom/response.rs

[highfive]

Warning

• These commits modify script code, but no tests are modified. Please consider adding a test!

[jdm]

@bors-servo r+
Thanks!

[bors-servo]

📌 Commit a744ac2 has been approved by jdm

[bors-servo]

⌛ Testing commit a744ac2 with merge 8dc59c6...

[bors-servo]

☀️ Test successful - checks-github, status-taskcluster
Approved by: jdm
Pushing 8dc59c6 to master...