Column.ToString override for returning the column name (#624)

Author: samueleresca

src/csharp/Microsoft.Spark.E2ETest/IpcTests/Sql/ColumnTests.cs

@@ -137,7 +137,10 @@ public void TestSignaturesV2_3_X()
             Assert.IsType<Column>(col1.BitwiseXOR(col2));
 
             Assert.IsType<Column>(col1.Over(PartitionBy(col1)));
-            Assert.IsType<Column>(col1.Over());          
+            Assert.IsType<Column>(col1.Over());
+            
+            Assert.Equal("col1", col1.ToString());
+            Assert.Equal("col2", col2.ToString());
         }
 
     }

src/csharp/Microsoft.Spark/Sql/Column.cs

@@ -886,6 +886,12 @@ public override bool Equals(object obj)
         /// <returns>Hash code for this object</returns>
         public override int GetHashCode() => _jvmObject.GetHashCode();
 
+        /// <summary>
+        /// Invoke the toString method of the column instance
+        /// </summary>
+        /// <returns>Column name of this column</returns>
+        public override string ToString() => (string)_jvmObject.Invoke("toString");
+
         /// <summary>
         /// Invokes a method under "org.apache.spark.sql.functions" with the given column.
         /// </summary>