Add 'isValid' method

baggagecontext/api/src/main/java/brown/tracingplane/BaggageProvider.java

@@ -10,6 +10,12 @@ import java.nio.ByteBuffer;
  */
 public interface BaggageProvider<B extends BaggageContext> {
 
+    /**
+     * @param baggage a BaggageContext to test
+     * @return true if <code>baggage</code> is an instance of {@link B} or if baggage is null.
+     */
+    boolean isValid(BaggageContext baggage);
+
     /**
      * @return a new instance of {@link B}, which may be null to indicate an empty baggage
      */

baggagecontext/noop-impl/src/main/java/brown/tracingplane/noopprovider/NoOpBaggageContextProvider.java

@@ -9,6 +9,11 @@ import brown.tracingplane.BaggageProvider;
  */
 public class NoOpBaggageContextProvider implements BaggageProvider<BaggageContext> {
 
+    @Override
+    public boolean isValid(BaggageContext baggage) {
+        return baggage == null;
+    }
+
     @Override
     public BaggageContext newInstance() {
         return null;