Hotfix: return correct error code after kill_server

Author: pcboy

lib/stripe_mock/api/server.rb

@@ -21,7 +21,14 @@ def spawn_server(opts={})
  ){
  StripeMock::Server.start_new(opts)
  }
- at_exit { kill_server(pid_path) }
+ at_exit {
+ begin
+ e = $! # last exception
+ kill_server(pid_path)
+ ensure
+ raise e if $! != e
+ end
+ }
  end
 
  def kill_server(pid_path=nil)