Do not force current time on 'editevent' request

The date and time arguments (year, mon, day, hour, min) are not actually required on [editevent](http://www.livejournal.com/doc/server/ljp.csp.xml-rpc.editevent.html) request. They should only be set if changed. 

But the current implementation forces sending the current date and time with `editevent`. If I want to update an old entry it pops up as the most recent one.

My patch allows the client not to send date and time arguments with `editevent`. Of course, if `entry.time` is set explicitly it is sent. And `postevent` sends current time by default as before.