class Faraday::HttpCache

def validate(entry, env)

Returns the 'Faraday::HttpCache::Response' to be forwarded into the stack.

env - the environment 'Hash' to perform the request.
entry - a stale 'Faraday::HttpCache::Response' retrieved from the cache.

response will be stored (replacing the old one) and used.
and forwarded against the Faraday stack. Otherwise, the freshly new
is marked as 'Not Modified', the previous stored response will be used
existing 'Last-Modified' and 'ETag' headers. If the new response
using the 'If-Modified-Since' and 'If-None-Match' headers with the
Internal: Tries to validated a stored entry back to it's origin server 
def validate(entry, env)
  headers = env[:request_headers]
  headers['If-Modified-Since'] = entry.last_modified if entry.last_modified
  headers['If-None-Match'] = entry.etag if entry.etag
  @app.call(env).on_complete do |requested_env|
    response = Response.new(requested_env)
    if response.not_modified?
      trace :valid
      updated_response_headers = response.payload[:response_headers]
      # These headers are not allowed in 304 responses, yet some proxy
      # servers add them in. Don't override the values from the original
      # response.
      updated_response_headers.delete('Content-Type')
      updated_response_headers.delete('Content-Length')
      updated_payload = entry.payload
      updated_payload[:response_headers].update(updated_response_headers)
      requested_env.update(updated_payload)
      response = Response.new(updated_payload)
    else
      trace :invalid
    end
    store(response)
  end
end