OFHTTPServer: Always send Server and Date.

This also makes sure those aren't sent twice in case they are also in
the headers.
OFObject: Cosmetic changes.
OFStream: Rename -[write*:usingEncoding:].

It is now -[write*:encoding:].
OFString: Add -[writeToFile:encoding:].
-[OFString cStringWithEncoding:]: Add ISO 8859-15.