Only add parameters to ListDomains request when required. This was necessary because commit 61df434296f5c084c08b2253a490623f5596327a changed the way that query strings are constructed in erlcloud_http.erl make_query_string/1, which broke the list_domains requests in erlcloud_sdb.erl when FirstToken was the empty string.
Merge pull request #72 from cobusc/master

Only add parameters to ListDomains request when required.