Use `Time#httpdate` instead of `Time#rfc822`

[kyanny]

Hi, thank you for making useful gem :)

I've made a tiny update for README.md.

Time#rfc822 (alias of Time#rfc2822) represents time zone as [+-]hhmm format. While Time#httpdate represents time zone as GMT always, which is preferable as HTTP Date header format.

example:

irb(main):001:0> require 'time'
=> true
irb(main):002:0> Time.now.rfc822
=> "Mon, 10 Aug 2020 17:03:58 +0900"
irb(main):003:0> Time.now.httpdate
=> "Mon, 10 Aug 2020 08:04:02 GMT"

references:

• https://tools.ietf.org/html/rfc2616#section-3.3
• https://tools.ietf.org/html/rfc7231#section-7.1.1.2
• https://ruby-doc.org/stdlib-2.7.1/libdoc/time/rdoc/Time.html#method-i-httpdate
• https://ruby-doc.org/stdlib-2.7.1/libdoc/time/rdoc/Time.html#method-i-rfc2822