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)
```
In Pg and Sqlite3, `:text` and `:binary` have variable unlimited length.
But in MySQL, these have limited length for each types (ref #21591, #21619).
This change adds short-hand methods for each text and blob types.
Example:
create_table :foos do |t|
t.tinyblob :tiny_blob
t.mediumblob :medium_blob
t.longblob :long_blob
t.tinytext :tiny_text
t.mediumtext :medium_text
t.longtext :long_text
end
Currently `tinyblob` is dumped to `t.binary "tiny_blob", limit: 255`.
But `t.binary ... limit: 255` is generating SQL to `varchar(255)`.
It is incorrect. This commit fixes this problem.
Kir Shatrov