HTTPoison 1.8.0

• Add support to "maybe redirect" response #424 Thanks, @vereis !
• Fix request/1 spec #418 Thanks, @flurin !
• Bump hackney version to 1.17 #430 Thanks, @evancloutier !
• Update README #427 & #428 Thanks, @81dr and @iloveitaly !
• Misc doc changes #429 Thanks, @kianmeng !
• Document multipart with in-memory content #422 Thanks, @pdgonzalez872 !