Only change the method of requests that Jetty won't handle by default

Previously, JettyEmbeddedErrorHandler would change the method of every
request that is handles to GET. This was being done to work around
Jetty's error handling only dealing with GET, POST, and HEAD requests
by default. It had the unwanted side-effect of causing an error
response to a HEAD request having a body as, from the error handling's
perspective, it was a GET request.

This commit updates JettyEmbeddedErrorHandler to only set the method
on a request for which error handling is being performed if the method
isn't already one that will be handled, leaving the method of GET,
POST, and HEAD requests unchanged.

Unfortunately, short of implementing an HTTP client, this change cannot
be tested as the Apache HttpClient, OkHttp, and the JDK's
HttpURLConnection all silently drop the body of a response to a HEAD
request, preventing a test from asserting that a body hasn't been sent.

Closes gh-23551

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyEmbeddedErrorHandler.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -17,6 +17,9 @@
 package org.springframework.boot.web.embedded.jetty;
 
 import java.io.IOException;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
 
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
@@ -37,6 +40,8 @@ import org.eclipse.jetty.servlet.ErrorPageErrorHandler;
  */
 class JettyEmbeddedErrorHandler extends ErrorPageErrorHandler {
 
+	private static final Set<String> HANDLED_HTTP_METHODS = new HashSet<>(Arrays.asList("GET", "POST", "HEAD"));
+
 	@Override
 	public boolean errorPageForMethod(String method) {
 		return true;
@@ -45,7 +50,9 @@ class JettyEmbeddedErrorHandler extends ErrorPageErrorHandler {
 	@Override
 	public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response)
 			throws IOException {
-		baseRequest.setMethod("GET");
+		if (!HANDLED_HTTP_METHODS.contains(baseRequest.getMethod())) {
+			baseRequest.setMethod("GET");
+		}
 		super.doError(target, baseRequest, request, response);
 	}