Use FETCH FIRST for Oracle12 database and Arel Oracle12 visitor

also test `ROWNUM <=` for Oracle 11g or older version of Oracle and Oracle visitor
Oracle 12c database and Arel Oracle12 visitor supports better top N query.

Author: yahonda

activerecord/test/cases/finder_test.rb

@@ -562,9 +562,9 @@ def test_model_class_responds_to_last_bang
   end
 
   def test_take_and_first_and_last_with_integer_should_use_sql_limit
-    assert_sql(/LIMIT|ROWNUM <=/) { Topic.take(3).entries }
-    assert_sql(/LIMIT|ROWNUM <=/) { Topic.first(2).entries }
-    assert_sql(/LIMIT|ROWNUM <=/) { Topic.last(5).entries }
+    assert_sql(/LIMIT|ROWNUM <=|FETCH FIRST/) { Topic.take(3).entries }
+    assert_sql(/LIMIT|ROWNUM <=|FETCH FIRST/) { Topic.first(2).entries }
+    assert_sql(/LIMIT|ROWNUM <=|FETCH FIRST/) { Topic.last(5).entries }
   end
 
   def test_last_with_integer_and_order_should_keep_the_order

activerecord/test/cases/scoping/relation_scoping_test.rb

@@ -246,7 +246,7 @@ def test_merge_options
         devs = Developer.all
         sql = devs.to_sql
         assert_match "(salary = 80000)", sql
-        assert_match "LIMIT 10", sql
+        assert_match /LIMIT 10|ROWNUM <= 10|FETCH FIRST 10 ROWS ONLY/, sql
       end
     end
   end