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.
Merge pull request #402 from bgilbert/gzip-timestamp

Set timestamps of files written by GzipCompressorFileStorage