Add test to column alias in exists? SQL.

This behavior was added in be4ecdc.

Closes #1139.

Fixes #2553, #1141, #1623 and #2062.

activerecord/lib/active_record/relation/finder_methods.rb

@@ -106,7 +106,7 @@ def first!
     #   Person.last # returns the last object fetched by SELECT * FROM people
     #   Person.where(["user_name = ?", user_name]).last
     #   Person.order("created_on DESC").offset(5).last
-    #   Person.last(3) # returns the last three objects fetched by SELECT * FROM people. 
+    #   Person.last(3) # returns the last three objects fetched by SELECT * FROM people.
     #
     # Take note that in that last case, the results are sorted in ascending order:
     #

activerecord/test/cases/finder_test.rb

@@ -45,6 +45,12 @@ def test_exists
     assert_raise(NoMethodError) { Topic.exists?([1,2]) }
   end
 
+  def test_exists_does_not_select_columns_without_alias
+    assert_sql(/SELECT\W+1 AS _one FROM ["`]topics["`]\W+LIMIT 1/) do
+      Topic.exists?
+    end
+  end
+
   def test_exists_returns_true_with_one_record_and_no_args
     assert Topic.exists?
   end