[SPARK-11884] Drop multiple columns in the DataFrame API

See the thread Ben started:
http://search-hadoop.com/m/q3RTtveEuhjsr7g/

This PR adds drop() method to DataFrame which accepts multiple column names

Author: tedyu <yuzhihong@gmail.com>

Closes #9862 from ted-yu/master.

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

@@ -1261,16 +1261,24 @@ class DataFrame private[sql](
    * @since 1.4.0
    */
   def drop(colName: String): DataFrame = {
+    drop(Seq(colName) : _*)
+  }
+
+  /**
+   * Returns a new [[DataFrame]] with columns dropped.
+   * This is a no-op if schema doesn't contain column name(s).
+   * @group dfops
+   * @since 1.6.0
+   */
+  @scala.annotation.varargs
+  def drop(colNames: String*): DataFrame = {
     val resolver = sqlContext.analyzer.resolver
-    val shouldDrop = schema.exists(f => resolver(f.name, colName))
-    if (shouldDrop) {
-      val colsAfterDrop = schema.filter { field =>
-        val name = field.name
-        !resolver(name, colName)
-      }.map(f => Column(f.name))
-      select(colsAfterDrop : _*)
-    } else {
+    val remainingCols =
+      schema.filter(f => colNames.forall(n => !resolver(f.name, n))).map(f => Column(f.name))
+    if (remainingCols.size == this.schema.size) {
       this
+    } else {
+      this.select(remainingCols: _*)
     }
   }
 

sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala

@@ -378,6 +378,13 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
     assert(df.schema.map(_.name) === Seq("value"))
   }
 
+  test("drop columns using drop") {
+    val src = Seq((0, 2, 3)).toDF("a", "b", "c")
+    val df = src.drop("a", "b")
+    checkAnswer(df, Row(3))
+    assert(df.schema.map(_.name) === Seq("c"))
+  }
+
   test("drop unknown column (no-op)") {
     val df = testData.drop("random")
     checkAnswer(