Correctly dump native timestamp types for MySQL

The native timestamp type in MySQL is different from datetime type.
Internal representation of the timestamp type is UNIX time, This means
that timestamp columns are affected by time zone.

```
> SET time_zone = '+00:00';
Query OK, 0 rows affected (0.00 sec)

> INSERT INTO time_with_zone(ts,dt) VALUES (NOW(),NOW());
Query OK, 1 row affected (0.02 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 22:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)

> SET time_zone = '-08:00';
Query OK, 0 rows affected (0.00 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 14:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)
```

1 files changed, 11 insertions, 1 deletions

diff --git a/activerecord/test/cases/defaults_test.rb b/activerecord/test/cases/defaults_test.rb
index 6532efcf22..a6297673c9 100644
--- a/activerecord/test/cases/defaults_test.rb
+++ b/activerecord/test/cases/defaults_test.rb
@@ -100,11 +100,21 @@ if current_adapter?(:Mysql2Adapter)
     include SchemaDumpingHelper
 
     if ActiveRecord::Base.connection.version >= "5.6.0"
-      test "schema dump includes default expression" do
+      test "schema dump datetime includes default expression" do
         output = dump_table_schema("datetime_defaults")
         assert_match %r/t\.datetime\s+"modified_datetime",\s+default: -> { "CURRENT_TIMESTAMP" }/, output
       end
     end
+
+    test "schema dump timestamp includes default expression" do
+      output = dump_table_schema("timestamp_defaults")
+      assert_match %r/t\.timestamp\s+"modified_timestamp",\s+default: -> { "CURRENT_TIMESTAMP" }/, output
+    end
+
+    test "schema dump timestamp without default expression" do
+      output = dump_table_schema("timestamp_defaults")
+      assert_match %r/t\.timestamp\s+"nullable_timestamp"$/, output
+    end
   end
 
   class DefaultsTestWithoutTransactionalFixtures < ActiveRecord::TestCase