Don't implicitly check preconditions on PUT requests

Prior to this commit, the `HttpEntityMethodProcessor` would check HTTP
conditions on non-safe requests (i.e. not GET/HEAD). This would prevent
Controllers from returning `ResponseEntity` containing response headers
with updated values of `"Last-Modified"` or `ETag` once the resource has
been updated.

This commit avoids those checks for non GET/HEAD requests - this code
can still be leveraged from Controllers themselves using
`ServletWebRequest::checkNotModified` methods.

Issue: SPR-15780
(cherry picked from ed7684d2b2)

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/HttpEntityMethodProcessorMockTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -511,6 +511,21 @@ public class HttpEntityMethodProcessorMockTests {
 		assertConditionalResponse(HttpStatus.OK, "body", etagValue, -1);
 	}
 
+	@Test
+	public void shouldNotFailPreconditionForPutRequests() throws Exception {
+		servletRequest.setMethod("PUT");
+		long dateTime = new Date().getTime();
+		servletRequest.addHeader(HttpHeaders.IF_UNMODIFIED_SINCE, dateFormat.format(dateTime));
+
+		long justModified = dateTime + 1;
+		ResponseEntity<String> returnValue = ResponseEntity.ok()
+				.lastModified(justModified).body("body");
+		initStringMessageConversion(MediaType.TEXT_PLAIN);
+		processor.handleReturnValue(returnValue, returnTypeResponseEntity, mavContainer, webRequest);
+
+		assertConditionalResponse(HttpStatus.OK, null, null, justModified);
+	}
+
 	@Test
 	public void varyHeader() throws Exception {
 		String[] entityValues = {"Accept-Language", "User-Agent"};