Set timestamps of files written by GzipCompressorFileStorage

nginx's HttpGzipStaticModule sets the Last-Modified response header from the timestamp of the precompressed .gz file, if it is used.  If the .gz file is created in a different second than the base file, this will cause the Last-Modified response header to vary depending on the Accept-Encoding request header.  Fix this by explicitly setting the timestamps of the base and .gz files.