418625 - WebSocket / Jsr RemoteEndpoint.sendObject(java.nio.HeapByteBuffer) doesn't find encoder

+ Adding fallback lookup using isAssignable(type) if basic map based
  lookup fails first.  Remembers successful isAssignable(type) match
  for future lookups via basic map approach.

jetty-websocket/javax-websocket-client-impl/src/main/java/org/eclipse/jetty/websocket/jsr356/metadata/CoderMetadataSet.java

@@ -18,6 +18,7 @@
 
 package org.eclipse.jetty.websocket.jsr356.metadata;
 
+import java.nio.ByteBuffer;
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
@@ -144,12 +145,37 @@ public abstract class CoderMetadataSet<T, M extends CoderMetadata<T>> implements
     {
         Integer idx = typeMap.get(type);
         if (idx == null)
+        {
+            // Quick lookup failed, try slower lookup via isAssignable instead
+            idx = getMetadataByAssignableType(type);
+            if (idx != null)
+            {
+                // add new entry map
+                typeMap.put(type,idx);
+            }
+        }
+
+        // If idx is STILL null, we've got no match
+        if (idx == null)
         {
             return null;
         }
         return metadatas.get(idx);
     }
 
+    private Integer getMetadataByAssignableType(Class<?> type)
+    {
+        for (Map.Entry<Class<?>, Integer> entry : typeMap.entrySet())
+        {
+            if (entry.getKey().isAssignableFrom(type))
+            {
+                return entry.getValue();
+            }
+        }
+
+        return null;
+    }
+
     @Override
     public Iterator<M> iterator()
     {