Restore original readOnlyHttpHeaders signature for binary compatibility

Closes gh-25034

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -383,7 +383,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 * An empty {@code HttpHeaders} instance (immutable).
 	 * @since 5.0
 	 */
-	public static final HttpHeaders EMPTY = new ReadOnlyHttpHeaders(new HttpHeaders(new LinkedMultiValueMap<>(0)));
+	public static final HttpHeaders EMPTY = new ReadOnlyHttpHeaders(new LinkedMultiValueMap<>());
 
 	/**
 	 * Pattern matching ETag multiple field values in headers such as "If-Match", "If-None-Match".
@@ -1085,6 +1085,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 	/**
 	 * Return the value of the {@code If-Match} header.
+	 * @throws IllegalArgumentException if parsing fails
 	 * @since 4.3
 	 */
 	public List<String> getIfMatch() {
@@ -1144,6 +1145,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 	/**
 	 * Return the value of the {@code If-None-Match} header.
+	 * @throws IllegalArgumentException if parsing fails
 	 */
 	public List<String> getIfNoneMatch() {
 		return getETagValuesAsList(IF_NONE_MATCH);
@@ -1471,6 +1473,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 * Retrieve a combined result from the field values of the ETag header.
 	 * @param headerName the header name
 	 * @return the combined result
+	 * @throws IllegalArgumentException if parsing fails
 	 * @since 4.3
 	 */
 	protected List<String> getETagValuesAsList(String headerName) {
@@ -1699,17 +1702,21 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 
 	/**
-	 * Apply a read-only {@code HttpHeaders} wrapper around the given headers.
+	 * Apply a read-only {@code HttpHeaders} wrapper around the given headers,
+	 * if necessary.
+	 * @param headers the headers to expose
+	 * @return a read-only variant of the headers, or the original headers as-is
 	 */
-	public static HttpHeaders readOnlyHttpHeaders(MultiValueMap<String, String> headers) {
+	public static HttpHeaders readOnlyHttpHeaders(HttpHeaders headers) {
 		Assert.notNull(headers, "HttpHeaders must not be null");
-		return (headers instanceof ReadOnlyHttpHeaders ?
-				(HttpHeaders) headers : new ReadOnlyHttpHeaders(headers));
+		return (headers instanceof ReadOnlyHttpHeaders ? headers : new ReadOnlyHttpHeaders(headers.headers));
 	}
 
 	/**
 	 * Remove any read-only wrapper that may have been previously applied around
-	 * the given headers via {@link #readOnlyHttpHeaders(MultiValueMap)}.
+	 * the given headers via {@link #readOnlyHttpHeaders(HttpHeaders)}.
+	 * @param headers the headers to expose
+	 * @return a writable variant of the headers, or the original headers as-is
 	 * @since 5.1.1
 	 */
 	public static HttpHeaders writableHttpHeaders(HttpHeaders headers) {