::CompositeIO and ::Parts deprecation warning

[konami99]

I'm using fitgem_oauth2 (3.0.0) and it depends on faraday (1.0.1), which depends on multipart-post (2.3.0).

multipart-post 2.3.0 is emitting this warning

Top level ::CompositeIO is deprecated, require 'multipart/post' and use `Multipart::Post::CompositeReadIO` instead!
Top level ::Parts is deprecated, require 'multipart/post' and use `Multipart::Post::Parts` instead!

The warning is related to how faraday uses multipart-post (lib/faraday/file_part.rb)

...
# @return [IO]
FilePart = ::UploadIO

# Multipart value used to POST a file.
#
# @deprecated Use FilePart instead of this class. It behaves identically, with
#   a matching name to ParamPart.
UploadIO = ::UploadIO
...

Starting from faraday 1.9.0, the team has removed this part and opted-in for another library faraday-multipart (see change log lostisland/faraday@v1.8.0...v1.9.0) and therefore has fixed the warning. So I'm bumping faraday to 1.9.0.