Properly provide the query string for group milestone filtering

We noticed today that the CE upstream MRs were getting the %10.9 milestone, despite that being closed, and us filtering on active. Turns out we were using HTTParty incorrectly.

cb7837f0 updates the client and specs to fix the usage.

d1be369d updates the logic so that when there are two milestones that can be considered "current", such as this period where we're after the start date of 10.8 and 11.0 but before the due date for both, we use the one with the most distant due date (11.0 in this case).

cc @leipert