Replace if exists with table_exists? and drop table statement wit…

…h `drop_table`

since 'drop table if exists' statement does not always work with some databases such as Oracle.
also Oracle drop table statement will not drop sequence objects.

activerecord/test/cases/migration/references_foreign_key_test.rb

@@ -10,8 +10,8 @@ class ReferencesForeignKeyTest < ActiveRecord::TestCase
       end
 
       teardown do
-        @connection.execute("drop table if exists testings")
-        @connection.execute("drop table if exists testing_parents")
+        @connection.drop_table("testings") if @connection.table_exists? "testings"
+        @connection.drop_table("testing_parents") if @connection.table_exists? "testing_parents"
       end
 
       test "foreign keys can be created with the table" do

activerecord/test/cases/primary_keys_test.rb

@@ -210,7 +210,7 @@ class Barcode < ActiveRecord::Base
   end
 
   teardown do
-    @connection.execute("DROP TABLE IF EXISTS barcodes")
+    @connection.drop_table(:barcodes) if @connection.table_exists? :barcodes
   end
 
   def test_any_type_primary_key