Problem in MacRuby somewhere...but where?

test.rb

require 'json'
require 'rubygems'
require 'restclient'

# setup db
DB = 'http://127.0.0.1:5984/test'
RestClient.delete DB rescue nil
RestClient.put DB, {}.to_json

# setup a document
doc  = { '_id' => 'test' }
resp = JSON.parse RestClient.put("#{DB}/test", doc.to_json)

# prepare the document
image = IO.read '/Applications/Mail.app/Contents/Resources/AccountPreferences.tiff'

# put it in the db
RestClient.put(
               "#{DB}/test/#{resp['_id']}/test.tiff?rev=#{resp['rev']}",
               image,
               content_type:'image/tiff'
               )

This works with CRuby, but not on MacRuby. I haven't figured out why yet, but I think it losing some informating somewhere in net/http when it makes the PUT request.