Fix bug with non-existing variable in Drop.create

When uploading a file via CloudApp::Drop.create the local variable "res"
is not initialized in case the initial request to /items/new returned an error.

lib/cloudapp/drop.rb

@@ -104,7 +104,7 @@ def self.create(kind, opts = {})
         res = post "/items", {:body => {:items => opts}, :digest_auth => @@auth}
       when :upload
         r = get "/items/new", {:query => ({:item => {:private => opts[:private]}} if opts.has_key?(:private)), :digest_auth => @@auth}
-        return bad_response(res) unless r.ok?
+        return bad_response(r) unless r.ok?
         res = post r['url'], Multipart.new(r['params'].merge!(:file => File.new(opts[:file]))).payload.merge!(:digest_auth => @@auth)
       else
         # TODO raise an error