Bug fix: cross-origin XHR with "Origin" or "Referrer" header set always return 403. 

cross-origin POST with "Origin" or "referrer" header from different domains will always returns 403. This is actually a bug from sinatra 1.3.2(https://github.com/sinatra/sinatra/issues/518), and upgrading it to any version from 1.3.3-1.4.2 fix the problem. 

This solves the issue that I created in https://github.com/beefproject/beef/issues/886