HOTFIX: ClassCastException in request logging

Author: Jason Gustafson <jason@confluent.io>

Reviewers: Colin P. Mccabe <cmccabe@confluent.io>, Ewen Cheslack-Postava <me@ewencp.org>

Closes #2566 from hachikuji/hotfix-request-logging

core/src/main/scala/kafka/network/RequestChannel.scala

@@ -108,7 +108,7 @@ object RequestChannel extends Logging {
       if (requestObj != null)
         requestObj.describe(details)
       else
-        s"$header -- $body"
+        s"$header -- ${body[AbstractRequest]}"
     }
 
     def body[T <: AbstractRequest : ClassTag] = {