Ignore HEAD requests in ShallowEtagHeaderFilter

Prior to this commit, the `ShallowEtagHeaderFilter` could participate in
the response and set its ETag/Content-Length headers, even for HEAD
requests. Since the response body is empty, the filter implementation
would set a `"Content-Length: 0"`.

The RFC states that responses to HEAD requests should exhibit identical
response headers to GET (with the possible exception of payload related
headers such as Content-Length.

With this commit, `ShallowEtagHeaderFilter` now ignores HEAD requests
since the proper values may be set already for payload related headers
by the handler. The filter has no way to generate a proper ETag value
nor calculate the content length without the actual body.

Issue: SPR-15261

Author: bclozel

spring-web/src/main/java/org/springframework/web/filter/ShallowEtagHeaderFilter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -19,6 +19,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.PrintWriter;
+
 import javax.servlet.FilterChain;
 import javax.servlet.ServletException;
 import javax.servlet.ServletOutputStream;
@@ -168,8 +169,8 @@ protected boolean isEligibleForEtag(HttpServletRequest request, HttpServletRespo
 			int responseStatusCode, InputStream inputStream) {
 
 		String method = request.getMethod();
-		if (responseStatusCode >= 200 && responseStatusCode < 300 &&
-				(HttpMethod.GET.matches(method) || HttpMethod.HEAD.matches(method))) {
+		if (responseStatusCode >= 200 && responseStatusCode < 300
+				&& HttpMethod.GET.matches(method)) {
 
 			String cacheControl = response.getHeader(HEADER_CACHE_CONTROL);
 			if (cacheControl == null || !cacheControl.contains(DIRECTIVE_NO_STORE)) {

spring-web/src/test/java/org/springframework/web/filter/ShallowEtagHeaderFilterTests.java

@@ -46,6 +46,9 @@ public void isEligibleForEtag() {
 		assertTrue(filter.isEligibleForEtag(request, response, 200, StreamUtils.emptyInput()));
 		assertFalse(filter.isEligibleForEtag(request, response, 300, StreamUtils.emptyInput()));
 
+		request = new MockHttpServletRequest("HEAD", "/hotels");
+		assertFalse(filter.isEligibleForEtag(request, response, 200, StreamUtils.emptyInput()));
+
 		request = new MockHttpServletRequest("POST", "/hotels");
 		assertFalse(filter.isEligibleForEtag(request, response, 200, StreamUtils.emptyInput()));