Fix @RequestBody argument processing for null Content-Type

Since the changes introduced in SPR-12778, some `@RequestBody` args
would not be properly processed in some cases:

* requests with an empty body
* no Content-Type header defined

This typically happens when GET requests are mapped on a handler dealing
with POST requests and HTTP bodies.

This change makes sure that the `RequestResponseBodyMethodProcessor` is
only involved for requests that:

* have a Content-Type defined
* OR are HTTP requests eligible for an HTTP body (PUT, POST, PATCH)

Issue: SPR-13176
Fixes spring-projects/spring-boot#3313

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

@@ -212,6 +212,7 @@ public class RequestMappingHandlerAdapterIntegrationTests {
 
 		Class<?>[] parameterTypes = new Class<?>[] { byte[].class };
 
+		request.setMethod("POST");
 		request.addHeader("Content-Type", "text/plain; charset=utf-8");
 		request.setContent("Hello Server".getBytes("UTF-8"));
 

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

@@ -102,6 +102,7 @@ public class RequestResponseBodyMethodProcessorMockTests {
 		mavContainer = new ModelAndViewContainer();
 
 		servletRequest = new MockHttpServletRequest();
+		servletRequest.setMethod("POST");
 		webRequest = new ServletWebRequest(servletRequest, new MockHttpServletResponse());
 	}
 
@@ -178,17 +179,11 @@ public class RequestResponseBodyMethodProcessorMockTests {
 		processor.resolveArgument(paramRequestBodyString, mavContainer, webRequest, null);
 	}
 
-	@Test
+	@Test(expected = HttpMediaTypeNotSupportedException.class)
 	public void resolveArgumentNoContentType() throws Exception {
 		servletRequest.setContent("payload".getBytes(Charset.forName("UTF-8")));
 		given(messageConverter.canRead(String.class, MediaType.APPLICATION_OCTET_STREAM)).willReturn(false);
-		try {
-			processor.resolveArgument(paramRequestBodyString, mavContainer, webRequest, null);
-			fail("Expected exception");
-		}
-		catch (HttpMediaTypeNotSupportedException ex) {
-			// expected
-		}
+		processor.resolveArgument(paramRequestBodyString, mavContainer, webRequest, null);
 	}
 
 	@Test(expected = HttpMediaTypeNotSupportedException.class)
@@ -217,6 +212,14 @@ public class RequestResponseBodyMethodProcessorMockTests {
 		assertNull(processor.resolveArgument(paramStringNotRequired, mavContainer, webRequest, new ValidatingBinderFactory()));
 	}
 
+	@Test
+	public void resolveArgumentNotGetRequests() throws Exception {
+		servletRequest.setMethod("GET");
+		servletRequest.setContent(new byte[0]);
+		given(messageConverter.canRead(String.class, MediaType.APPLICATION_OCTET_STREAM)).willReturn(false);
+		assertNull(processor.resolveArgument(paramStringNotRequired, mavContainer, webRequest, new ValidatingBinderFactory()));
+	}
+
 	@Test
 	public void handleReturnValue() throws Exception {
 		MediaType accepted = MediaType.TEXT_PLAIN;

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

@@ -112,6 +112,7 @@ public class RequestResponseBodyMethodProcessorTests {
 		mavContainer = new ModelAndViewContainer();
 
 		servletRequest = new MockHttpServletRequest();
+		servletRequest.setMethod("POST");
 		servletResponse = new MockHttpServletResponse();
 		webRequest = new ServletWebRequest(servletRequest, servletResponse);
 
@@ -140,6 +141,7 @@ public class RequestResponseBodyMethodProcessorTests {
 	@Test
 	public void resolveArgumentRawTypeFromParameterizedType() throws Exception {
 		String content = "fruit=apple&vegetable=kale";
+		this.servletRequest.setMethod("GET");
 		this.servletRequest.setContent(content.getBytes("UTF-8"));
 		this.servletRequest.setContentType(MediaType.APPLICATION_FORM_URLENCODED_VALUE);