Methods of Data::Page accept only scalars.

This is a fix that prevent making mistake.

Here is the sample script:

```perl
my $total_entries = { a => 123 }; # <--- Here is the error! It should be number
my $entries_per_page = 10; 
my $current_page = 1;

my $page = Data::Page->new();
$page->total_entries($total_entries);
$page->entries_per_page($entries_per_page);
$page->current_page($current_page);

print "         First page: ", $page->first_page, "\n";
print "          Last page: ", $page->last_page, "\n";
print "First entry on page: ", $page->first, "\n";
print " Last entry on page: ", $page->last, "\n";
```

It prints something like this:
```
         First page: 1
          Last page: 652424
First entry on page: 1
 Last entry on page: 10
```

The problem is with the last page value.

PS This issue was originally discussed on http://play-perl.org/quest/51278657e8e8478d05000016