Fix parsing of timezones with underscores

Fixes http://framework.zend.com/issues/browse/ZF-11552

Zend_Date does not correctly parse timezones which contain an underscore. Eg: America/New_York

Non-underscored timezones are parsed correctly. Eg: America/Denver

```php
$date = new Zend_Date('2011-01-01 00:00:00 UTC');
echo $date->getTimezone()."\n"; // Correctly prints "UTC"

$date = new Zend_Date('2011-01-01 00:00:00 America/Denver');
echo $date->getTimezone()."\n"; // Correctly prints "America/Denver"

$date = new Zend_Date('2011-01-01 00:00:00 America/New_York');
echo $date->getTimezone()."\n"; // Incorrectly prints "UTC"

$date = new Zend_Date('2011-01-01 00:00:00 America/Cambridge_Bay');
echo $date->getTimezone()."\n";  // Incorrectly prints "UTC"
```