https://github.com/bblimke/webmock/issues/307

excon.rb

require 'excon'

# require 'webmock'
# WebMock.allow_net_connect!

s = StringIO.new("muffins")

resp = Excon.put("http://localhost:9292", request_block: lambda { s.read(2).to_s })

puts resp.body

server.rb

require 'rack'

app = lambda { |env|
  [200, {"Content-Type" => "text/plain"}, ["Got: #{env['rack.input'].read}"]]
}

trap('INT') {
  Rack::Handler::WEBrick.shutdown
}

Rack::Handler::WEBrick.run(app, :Port => 9292)