Fix pagination with string params

Fixes https://github.com/boto/botocore/issues/243 by making sure the max items pagination param uses the same type as the parameter it is replacing.

```bash
$ aws route53 list-resource-record-sets --hosted-zone-id /hostedzone/ABCD --max-items 1

A client error (NoSuchHostedZone) occurred when calling the ListResourceRecordSets operation: No hosted zone found with ID: ABCD
```

@jamesls please review.