Reset the file's timestamp to the current time on file.save

I use services in conjunction with deploy module to deploy content between environments. Just had a nasty thing happen in a production environment as a result of the following set of circumstances:
1. Files got deployed as dependencies of nodes and were saved on remote with status 0 and the timestamp from when they were originally created in a previous environment
2. Cron ran before the nodes got pushed so all the files got deleted because they were older than 6 hours (based on their timestamps) and had status 0
3. Nodes got pushed, but their files were already gone.

The attached patch ensures that the file is saved with the current time (i.e. on the remote, at the time of saving) instead of the original time that the node was created in a different environment.

I've set the priority of this as major because its consequences were disastrous in this particular instance. I don't know of any use case where hanging on to an old date that is relevant to a different environment is desirable.