Require 'date' to avoid retry exception

Without requiring 'date', we get an exception in the retry middleware
when it tries to use DateTime for handling the Retry-After header.

lib/faraday.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'cgi'
+require 'date'
 require 'set'
 require 'forwardable'
 require 'faraday/middleware_registry'