Raise visibility of FeignOutputMessage. Fixes gh-734.

spring-cloud-openfeign-core/src/main/java/org/springframework/cloud/openfeign/support/SpringEncoder.java

@@ -255,7 +255,7 @@ public class SpringEncoder implements Encoder {
 				.anyMatch(mediaType -> mediaType.includes(contentType));
 	}
 
-	private final class FeignOutputMessage implements HttpOutputMessage {
+	protected final class FeignOutputMessage implements HttpOutputMessage {
 
 		private final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();