Merge pull request ruby-ldap#18 from doitian/issue/17_bererror_unsupported_object_type_id_139

Do not add controls when it is empty. Fixed ruby-ldap#17

Author: RoryO

lib/net/ldap.rb

@@ -1394,9 +1394,9 @@ def search(args = {})
           false.to_ber,
           rfc2696_cookie.map{ |v| v.to_ber}.to_ber_sequence.to_s.to_ber
         ].to_ber_sequence if paged_searches_supported
-      controls = controls.to_ber_contextspecific(0)
+      controls = controls.empty? ? nil : controls.to_ber_contextspecific(0)
 
-      pkt = [next_msgid.to_ber, request, controls].to_ber_sequence
+      pkt = [next_msgid.to_ber, request, controls].compact.to_ber_sequence
       @conn.write pkt
 
       result_code = 0