Fix: Volumes paginated

[nicholi]

Noticed the Volumes list calls (/v2/volumes) returns the the paginated links section as relative URLs and not absolute.

  "links": {
    "pages": {
      "next": "/v2/volumes?page=2",
      "last": "/v2/volumes?page=10"
    }
  },

Is this a DO API bug? Or has it always been this way? We haven't used volumes up till this point, primarily Droplets. Which have always returned an absolute URL in their links section.

Slight fixes to generating next page call, will now safely allow both.