Relax generic type detection for ResponseEntity

Before this change the getHttpEntityType method in
HttpEntityMethodProcessor raised an ISE if the generic type cannot be
detected. That made sense for resolving a controller method argument
where the target body type is crucial. However for a return value
the generic type should not be required since we either have an
actual body or no body at all in which case it doesn't even matter.

This change relaxes the checks and defaults to Object.class for the
ResponseEntity generic type on the return value side.

Issue: SPR-14799

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/HttpEntityMethodProcessor.java

@@ -121,6 +121,10 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 
 		ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
 		Type paramType = getHttpEntityType(parameter);
+		if (paramType == null) {
+			throw new IllegalArgumentException("HttpEntity parameter '" + parameter.getParameterName() +
+					"' in method " + parameter.getMethod() + " is not parameterized");
+		}
 
 		Object body = readWithMessageConverters(webRequest, parameter, paramType);
 		if (RequestEntity.class == parameter.getParameterType()) {
@@ -146,8 +150,9 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 		else if (parameterType instanceof Class) {
 			return Object.class;
 		}
-		throw new IllegalArgumentException("HttpEntity parameter '" + parameter.getParameterName() +
-				"' in method " + parameter.getMethod() + " is not parameterized");
+		else {
+			return null;
+		}
 	}
 
 	@Override
@@ -243,6 +248,7 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 		}
 		else {
 			Type type = getHttpEntityType(returnType);
+			type = (type != null ? type : Object.class);
 			return ResolvableType.forMethodParameter(returnType, type).resolve(Object.class);
 		}
 	}