p3(http): Assert header immutability via delete, get_and_delete (#271)

This commit asserts that headers cannot be mutated via `delete` and
`get_and_delete`.

Author: saulecabrera

tests/rust/wasm32-wasip3/src/bin/http-request.rs

@@ -245,6 +245,15 @@ fn test_immutable_headers(headers: &Fields) {
         headers.append("Last-Modified", b"whatever"),
         Err(HeaderError::Immutable)
     );
+    assert_eq!(
+        headers.set("Last-Modified", &[b"whatever".to_vec()]),
+        Err(HeaderError::Immutable)
+    );
+    let all = headers.copy_all();
+    if let Some((name, _)) = all.first() {
+        assert_eq!(headers.delete(name), Err(HeaderError::Immutable));
+        assert_eq!(headers.get_and_delete(name), Err(HeaderError::Immutable));
+    }
 }
 
 fn test_headers_same(left: &Fields, right: &Fields) {

tests/rust/wasm32-wasip3/src/bin/http-response.rs

@@ -27,6 +27,15 @@ fn test_immutable_headers(headers: &Fields) {
         headers.append("Last-Modified", b"whatever"),
         Err(HeaderError::Immutable)
     );
+    assert_eq!(
+        headers.set("Last-Modified", &[b"whatever".to_vec()]),
+        Err(HeaderError::Immutable)
+    );
+    let all = headers.copy_all();
+    if let Some((name, _)) = all.first() {
+        assert_eq!(headers.delete(name), Err(HeaderError::Immutable));
+        assert_eq!(headers.get_and_delete(name), Err(HeaderError::Immutable));
+    }
 }
 
 fn test_headers_same(left: &Fields, right: &Fields) {