URI.encode @roles when sending request

This enables support for roles with spaces in the names.

fixes errors like the following:

    `merge': bad URI(is not URI?): /clients/XXXXXXXXXXX-PK/ping.scout?roles=Production Resque&fqdn=resque001.example.com (URI::InvalidURIError)

We were running into issues because our roles in the scout dashboard have spaces. A temporary workaround was to replace spaces with '+' in in the role names in our chef recipes, but this is a better long-term fix.