SPARK-6569 [STREAMING] Down-grade same-offset message in Kafka streaming to INFO

Reduce "is the same as ending offset" message to INFO level per JIRA discussion

Author: Sean Owen <sowen@cloudera.com>

Closes #5366 from srowen/SPARK-6569 and squashes the following commits:

8a5b992 [Sean Owen] Reduce "is the same as ending offset" message to INFO level per JIRA discussion

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(s"Beginning offset ${part.fromOffset} is the same as ending offset " +
+      log.info(s"Beginning offset ${part.fromOffset} is the same as ending offset " +
         s"skipping ${part.topic} ${part.partition}")
       Iterator.empty
     } else {