class Rack::Typhoeus::Middleware::ParamsDecoder

def update_params(req, k, v)

https://github.com/rack/rack/blob/master/lib/rack/request.rb#L243
Persist params change in environment. Extracted from: 
def update_params(req, k, v)
  found = false
  if req.GET.has_key?(k)
    found = true
    req.GET[k] = v
  end
  if req.POST.has_key?(k)
    found = true
    req.POST[k] = v
  end
  unless found
    req.GET[k] = v
  end
end