Use at_exit instead of overriding trap for interrup to shutdown Proxy…

…Server, AuthenticatedProxyServer.

Call to trap replaces handler for specified signal,
so ProxyServer.shutdown would have never been called.
at_exit registers handler for execution when the programm
exits, handlers are executed in revers order of registration.

spec/support/proxy_server.rb

@@ -18,14 +18,8 @@
   :RequestCallback => handler
 )
 
-Thread.new  { ProxyServer.start }
-trap("INT") do
-  ProxyServer.shutdown
-  exit
-end
+Thread.new { ProxyServer.start }
+at_exit { ProxyServer.shutdown }
 
-Thread.new  { AuthenticatedProxyServer.start }
-trap("INT") do
-  AuthenticatedProxyServer.shutdown
-  exit
-end
+Thread.new { AuthenticatedProxyServer.start }
+at_exit { AuthenticatedProxyServer.shutdown }