Replace timestmap 0 value with '0000-00-00 00:00:00'

Fixed BG-169 issue.

- TIMESTAMP data type(MySQL 5.5)

```
# Normal value
INSERT INTO masashi_test.timestamp_test VALUES (1, 915314592)
# 0 value
INSERT INTO masashi_test.timestamp_test VALUES (2, 0000-00-00 00:00:00)
```

- TIMESTAMP2 data type(MySQL 5.6)

```
# Normal value without fraction
INSERT INTO binlogtest.timestamp_test VALUES (1, 1403812196)

# Normal value with fraction
INSERT INTO masashi_test.timestamp2_test VALUES (13, 'asdafs', 347314861.000000)

# 0 without fraction
INSERT INTO masashi_test.timestamp_test VALUES (9, 'asdf', 0000-00-00 00:00:00)

# 0 with fraction
INSERT INTO masashi_test.timestamp1_test VALUES (2, 'asdf', 0000-00-00 00:00:00.0)
```
I'm not sure what the original value of str can be here, but I guess it's like /\d\.0/ because values such as "0.4" won't meet the if condition.
If so, should we better hard code ".0"?