Fix if unmodified pre 1.3.0

[yohanboniface]

Prior to 1.3.0, uMap was not setting the gzip mtime, so it was
whatever the time it get requested at first.
Since 1.3.0:

• when creating the geojson.gzip, we also force its mtime to be
  the geojson one
• we replaced If-Match by If-Unmodified, which relies on Last-Modified

When uMap is served by a proxy like Nginx (and X-Accel-Redirect),
and if user accepts gzip, then the Last-Modified would be the gzip
one, not the flat geojson one.

So when comparing that value in a subsequent update, we need to
compare with the correct value.

fix #1212