Fix edge case in ruby reaper

Closes #653

The problem was that the reaper returns nil instead of the memo. Many thanks to @francesmcmullin for the wonderfully detailed report and investigation. Now we have a regression test for this behavior.

lib/sidekiq_unique_jobs/orphans/ruby_reaper.rb

@@ -59,7 +59,7 @@ def orphans
           next if belongs_to_job?(digest)
 
           memo << digest
-          break if memo.size >= reaper_count
+          break memo if memo.size >= reaper_count
         end
       end
 

spec/sidekiq_unique_jobs/orphans/ruby_reaper_spec.rb

@@ -32,6 +32,28 @@
   describe "#orphans" do
     subject(:orphans) { service.orphans }
 
+    context "when reaping more jobs than reaper_count" do
+      let(:digest_one)   { "uniquejobs:digest1" }
+      let(:digest_two)   { "uniquejobs:digest2" }
+      let(:digest_three) { "uniquejobs:digest3" }
+      let(:job_id_one)   { "jobid1" }
+      let(:job_id_two)   { "jobid2" }
+      let(:job_id_three) { "jobid3" }
+
+      before do
+        SidekiqUniqueJobs::Lock.create(digest_one, job_id_one)
+        SidekiqUniqueJobs::Lock.create(digest_two, job_id_two)
+        SidekiqUniqueJobs::Lock.create(digest_three, job_id_three)
+      end
+
+      it 'returns the first digest' do
+        SidekiqUniqueJobs.use_config(reaper_count: 1) do
+          expect(orphans.size).to eq(1)
+          expect([digest_one, digest_two, digest_three]).to include(orphans.first)
+        end
+      end
+    end
+
     context "when scheduled" do
       let(:item) { raw_item.merge("at" => Time.now.to_f + 3_600) }