Image multipart upload

because oauth2 is using faraday gem to do the http request.
so we can upload image in faraday way.
the block passing to OAuth2::Client is the one to Faraday::Builder
your can read faraday#advanced-middleware-usage to understand how it work.
here are two key part to upload image.

client = OAuth2::Client.new(key, secret,
  :site => site, :authorize_url => auth_url, :token_url => token_url) do |b|
  b.request  :multipart
  b.request  :url_encoded
  b.adapter  :net_http
end

begin
  img = Faraday::UploadIO.new('play.gif', 'image/gif')
  data = {:content => "this is a tile", :img => img}
  response = token.post('api/v1/tiles/', :body => data)
  print response.body, response.status
rescue OAuth2::Error => e
  puts e
end