GridFS issues wrong update with a _closed attribute

When calling GridIn.close there's an interaction between self.closed = True and GridIn.setattr_. What this means is that right after closing GridIn there's an immediate and unnecessary update setting _closed field to true.

If it is intended for user code to be able to set closed (and other underscore-starting fields) to arbitrary values, then GridIn.close should be changed to use object.setattr_(self, '_closed', True).

If it was not intended then condition in _setattr_ should be changed to something like if self.closed and not name.startswith('')

I'm not sure about priority, but I set it to Major because it greatly affects performance (the update is done using safe=True), floods oplog with these unnecessary updates and so is very annoying.