v0.9.0.rc6

• Fix multipart content type values. (@lloydmeta)
• Better support for HTTP proxying. (@mislav)
• Make Faraday::Options behave more like a hash. (@mislav, @benburkert)
• Improved client cert configuration in Excon (@ehowe)
• Retry middleware adds backoff/randomness. (@timgaleckas)

⭐ See the full changelog ⭐