[SPARK-6296] [SQL] Added equals to Column

Author: Volodymyr Lyubinets <vlyubin@gmail.com>

Closes #4988 from vlyubin/columncomp and squashes the following commits:

92d7c8f [Volodymyr Lyubinets] Added equals to Column

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

@@ -59,6 +59,13 @@ class Column(protected[sql] val expr: Expression) {
 
   override def toString: String = expr.prettyString
 
+  override def equals(that: Any) = that match {
+    case that: Column => that.expr.equals(this.expr)
+    case _ => false
+  }
+
+  override def hashCode: Int = this.expr.hashCode
+
   /**
    * Unary minus, i.e. negate the expression.
    * {{{

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

@@ -313,4 +313,9 @@ class ColumnExpressionSuite extends QueryTest {
   test("lift alias out of cast") {
     assert(col("1234").as("name").cast("int").expr === col("1234").cast("int").as("name").expr)
   }
+
+  test("columns can be compared") {
+    assert('key.desc == 'key.desc)
+    assert('key.desc != 'key.asc)
+  }
 }