JPagination fix: Wrong pagination when used a multiple instances

This pull fix the issue when a couple JPagination instances used.

**Test**
Run next code:
```php
$p1 = new JPagination(10, 0, 5, 'pref1');
$data1 = $p1->getData();
$p2 = new JPagination(20, 0, 10, 'pref2');
$data2 = $p2->getData();
echo '<pre>';
print_r($data1->next);
print_r($data2->next);
echo '</pre>';
```

Result (before patch):
```
JPaginationObject Object
(
    [text] => Next
    [base] => 5
    [link] => /jdev/?pref1limitstart=5
    [prefix] => pref1
    [active] => 
)
JPaginationObject Object
(
    [text] => Next
    [base] => 5
    [link] => /jdev/?pref1limitstart=5
    [prefix] => pref1
    [active] => 
)
```

**Expected result:**

```
JPaginationObject Object
(
    [text] => Next
    [base] => 5
    [link] => /jdev/?pref1limitstart=5
    [prefix] => pref1
    [active] => 
)
JPaginationObject Object
(
    [text] => Next
    [base] => 10
    [link] => /jdev/?pref2limitstart=10
    [prefix] => pref2
    [active] => 
)
```