[SPARK-6279][Streaming]In KafkaRDD.scala, Miss expressions flag "s" at logging string

In KafkaRDD.scala, Miss expressions flag "s" at logging string
In logging file, it print `Beginning offset $
{part.fromOffset}
is the same as ending offset ` but not `Beginning offset 111 is the same as ending offset `.

Author: zzcclp <xm_zzc@sina.com>

Closes #4979 from zzcclp/SPARK-6279 and squashes the following commits:

768f88e [zzcclp] Miss expressions flag "s"

external/kafka/src/main/scala/org/apache/spark/streaming/kafka/KafkaRDD.scala

@@ -86,7 +86,7 @@ class KafkaRDD[
     val part = thePart.asInstanceOf[KafkaRDDPartition]
     assert(part.fromOffset <= part.untilOffset, errBeginAfterEnd(part))
     if (part.fromOffset == part.untilOffset) {
-      log.warn("Beginning offset ${part.fromOffset} is the same as ending offset " +
+      log.warn(s"Beginning offset ${part.fromOffset} is the same as ending offset " +
         s"skipping ${part.topic} ${part.partition}")
       Iterator.empty
     } else {