Fix for Database bug #2353

Table prefix is erroneously prefixed if select field contains a quoted string containing a period.

```
$this->db
    ->select('id, db.table.col, table.col, table.\'file_name.png\', table."file_name.png", "file_name.png", \'file.name\', "file"".name"', FALSE)
    ->from('prefix_table');

```

now yields:

```
SELECT id, db.prefix_table.col, prefix_table.col, prefix_table.'file_name.png', prefix_table."file_name.png", "file_name.png", 'file.name', "file"".name"
FROM `prefix_table`
```