Bug fix: set @open_connection to nil at the end of a Net::LDAP#open.

Otherwise, you couldn't do more than one #open on the same Net::LDAP
object! Subsequent ones would say "open already in progress."

lib/net/ldap.rb

@@ -543,6 +543,7 @@ module Net
       @open_connection.bind @auth
       yield self
       @open_connection.close
+      @open_connection = nil
     end