Fix @RequestBody(required=false) support

Prior to this commit, requests with an empty body and no Content-Type
header set would fail with a HttpMediaTypeNotSupportedException when
mapped to a Controller method argument annotated with
@RequestBody(required=false).
In those cases, the server implementation considers with an
"application/octet-stream" content type and polls messageconverters for
conversion. If no messageconverter is able to process this request, a
HttpMediaTypeNotSupportedException is thrown.

This change makes sure that such exceptions are not thrown if the
incoming request has:
* no body
* no content-type header

In this case, a null value is returned.

Issue: SPR-13147

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

@@ -158,6 +158,7 @@ public abstract class AbstractMessageConverterMethodArgumentResolver implements
 			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
 
 		MediaType contentType;
+		boolean noContentType = false;
 		try {
 			contentType = inputMessage.getHeaders().getContentType();
 		}
@@ -165,6 +166,7 @@ public abstract class AbstractMessageConverterMethodArgumentResolver implements
 			throw new HttpMediaTypeNotSupportedException(ex.getMessage());
 		}
 		if (contentType == null) {
+			noContentType = true;
 			contentType = MediaType.APPLICATION_OCTET_STREAM;
 		}
 
@@ -220,7 +222,8 @@ public abstract class AbstractMessageConverterMethodArgumentResolver implements
 		}
 
 		if (body == NO_VALUE) {
-			if (!SUPPORTED_METHODS.contains(httpMethod)) {
+			if (!SUPPORTED_METHODS.contains(httpMethod)
+					|| (noContentType && inputMessage.getBody() == null)) {
 				return null;
 			}
 			throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);