Fixed W3C tracestate and baggage setting

without this change we're not setting tracestate whatsover. Baggage is also not being propagated properly (via the baggage field)
with this change when tracestate is there in the headers it will be treated as baggage. When the context is propagated then tracestate will not be put to the baggage field

fixes gh-2001

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/BaggageManager.java

@@ -18,6 +18,8 @@ package org.springframework.cloud.sleuth;
 
 import java.util.Map;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Manages {@link BaggageInScope} entries. Upon retrieval / creation of a baggage entry
  * puts it in scope. Scope must be closed.
@@ -38,6 +40,7 @@ public interface BaggageManager {
 	 * @param name baggage name
 	 * @return baggage or {@code null} if not present
 	 */
+	@Nullable
 	BaggageInScope getBaggage(String name);
 
 	/**
@@ -46,6 +49,7 @@ public interface BaggageManager {
 	 * @param name baggage name
 	 * @return baggage or {@code null} if not present
 	 */
+	@Nullable
 	BaggageInScope getBaggage(TraceContext traceContext, String name);
 
 	/**