[SPARK-5514] DataFrame.collect should call executeCollect

Author: Reynold Xin <rxin@databricks.com>

Closes #4313 from rxin/SPARK-5514 and squashes the following commits:

e34e91b [Reynold Xin] [SPARK-5514] DataFrame.collect should call executeCollect

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -541,7 +541,7 @@ class DataFrame protected[sql](
   /**
    * Returns an array that contains all of [[Row]]s in this [[DataFrame]].
    */
-  override def collect(): Array[Row] = rdd.collect()
+  override def collect(): Array[Row] = queryExecution.executedPlan.executeCollect()
 
   /**
    * Returns a Java list that contains all of [[Row]]s in this [[DataFrame]].