FIX: handle heroku style HTTP_X_REQUEST_START (PR #10087)

Noticed search and other features were not working on our heroku instance after upgrade. A little sleuthing showed the issue:

$ rbtrace --ps "puma: cluster" -m 'Middleware::RequestTracker#rate_limit(request.env["REQUEST_QUEUE_SECONDS"])'
*** attached to process 78
Middleware::RequestTracker#rate_limit(request.env["REQUEST_QUEUE_SECONDS"]=1592524121.9201534) <0.002393>
Middleware::RequestTracker#rate_limit(request.env["REQUEST_QUEUE_SECONDS"]=1592524122.3583024) <0.002228>
Middleware::RequestTracker#rate_limit(request.env["REQUEST_QUEUE_SECONDS"]=1592524122.6266675) <0.002415>

after this PR:

Middleware::RequestTracker#rate_limit(request.env["REQUEST_QUEUE_SECONDS"]=0.009975671768188477) <0.006217>

GitHub

Thanks, looks good to me!