Differentiate request vs. response body

[mislav]

After status has been written for the first time, treat env[:body] as response body and save the previous value as request body.

$ RUBYLIB=./lib irb -rfaraday
>> env = Faraday::Env.from :body => 'REQUEST'
>> p env.request_body; env.response_body
"REQUEST"
nil
>> env[:status] = 200
>> env[:body] = 'RESPONSE'
>> p env.request_body; env.response_body
"REQUEST"
"RESPONSE"

[iMacTia]

Closing in favour of #847 which expands on this