Ensure clients set X-Not-Sampled header for non-exportable Span

It also turns out that there's a bug in Spring MVC which makes an
empty header blow up in RequestHeaderMapMethodArgumentResolver
so we set the header to "true" instead of empty.

Fixes gh-136

spring-cloud-sleuth-stream/src/main/java/org/springframework/cloud/sleuth/stream/SleuthStreamAutoConfiguration.java

@@ -62,7 +62,7 @@ public class SleuthStreamAutoConfiguration {
 			@Override
 			public Message<?> preSend(Message<?> message, MessageChannel channel) {
 				return MessageBuilder.fromMessage(message)
-						.setHeader(Span.NOT_SAMPLED_NAME, "").build();
+						.setHeader(Span.NOT_SAMPLED_NAME, "true").build();
 			}
 
 			@Override