HttpHeaders.writeableHttpHeaders should unwrap many times

Prior to this commit, the `HttpHeaders.writeableHttpHeaders` would only
consider headers read-only instances that were wrapped once by
`HttpHeaders.readOnlyHttpHeaders`. This does not work when other
`HttpHeaders` wrappers are involved in the chain.

This commit ensures that `writeableHttpHeaders` unwraps all headers
instances down to the actual multivalue map and create a new headers
instance out of it.

Fixes gh-33789

spring-web/src/test/java/org/springframework/http/HttpHeadersTests.java

@@ -57,6 +57,14 @@ class HttpHeadersTests {
 	private final HttpHeaders headers = new HttpHeaders();
 
 
+	@Test
+	void writableHttpHeadersUnwrapsMultiple() {
+		HttpHeaders originalExchangeHeaders = HttpHeaders.readOnlyHttpHeaders(new HttpHeaders());
+		HttpHeaders firewallHeaders = new HttpHeaders(originalExchangeHeaders);
+		HttpHeaders writeable = HttpHeaders.writableHttpHeaders(firewallHeaders);
+		writeable.setContentType(MediaType.APPLICATION_JSON);
+	}
+
 	@Test
 	void getOrEmpty() {
 		String key = "FOO";