fixing code review comments

Author: kevinyu98

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

@@ -1256,9 +1256,12 @@ class DataFrame private[sql](
   @scala.annotation.varargs
   def drop(colNames: String*): DataFrame = {
     val resolver = sqlContext.analyzer.resolver
+
     val remainingCols =
       schema.filter(f => colNames.forall(n => !resolver(f.name, n)))
-        .map(f => Column(UnresolvedAttribute(Seq(f.name))))
+        .map(f => Column(UnresolvedAttribute.quoted(f.name)))
+
+
     if (remainingCols.size == this.schema.size) {
       this
     } else {

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

@@ -36,11 +36,6 @@ import org.apache.spark.sql.types._
 class DataFrameSuite extends QueryTest with SharedSQLContext {
   import testImplicits._
 
-  test("SPARK-12987") {
-    val df = Seq((1, 1)).toDF("a_b", "a.c")
-    val df1 = df.drop("a_b")
-    assert(df1.toString() === "[a.c: int]")
-    }
 
   test("analysis error should be eagerly reported") {
     // Eager analysis.
@@ -1297,4 +1292,11 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
       Seq(1 -> "a").toDF("i", "j").filter($"i".cast(StringType) === "1"),
       Row(1, "a"))
   }
+
+  test("SPARK-12987: drop column ") {
+    val df = Seq((1, 2)).toDF("a_b", "a.c")
+    val df1 = df.drop("a_b")
+    checkAnswer(df1, Row(2))
+    assert(df1.schema.map(_.name) === Seq("a.c"))
+  }
 }