Put in an explicit expiration time and add spec

lib/gitlab/usage_data.rb

@@ -2,7 +2,7 @@ module Gitlab
   class UsageData
     class << self
       def data(force_refresh = false)
-        Rails.cache.fetch('usage_data', force: force_refresh) { uncached_data }
+        Rails.cache.fetch('usage_data', force: force_refresh, expires_in: 2.weeks) { uncached_data }
       end
 
       def uncached_data

spec/workers/gitlab_usage_ping_worker_spec.rb

@@ -8,6 +8,7 @@ describe GitlabUsagePingWorker do
 
     stub_request(:post, "https://version.gitlab.com/usage_data").
         to_return(status: 200, body: '', headers: {})
+    expect(Gitlab::UsageData).to receive(:to_json).with(true).and_call_original
     expect(subject).to receive(:try_obtain_lease).and_return(true)
 
     expect(subject.perform.response.code.to_i).to eq(200)