Oracle incorrect SELECT FROM decoration with table alias when using nested selects

Using the Oracle adapter, creating a SELECT query with more than 1 nested sub-select, like this:
```php
$sql = new \Zend\Db\Sql\Sql($adapter);

$select = $sql->select()
    ->from('test');

$select = $sql->select()
    ->from(array('a' => $select));

$select = $sql->select()
    ->from(array('b' => $select));

$sql->prepareStatementForSqlObject($select);
```

results in the following SQL:
```sql
SELECT "b".* FROM (SELECT "a".* FROM (SELECT "test".* FROM "test") AS "a") "b"
```

The expected SQL for Oracle is:
```sql
SELECT "b".* FROM (SELECT "a".* FROM (SELECT "test".* FROM "test") "a") "b"
```

Oracle only allows the use of AS for aliasing columns - not tables.

This bug is similar to #4069.