dataFileUpdateService: If-Modified-Since is not in GMT

[justadreamer]

Background

If you use a custom data File URL and you are not in GMT timezone - you will see that the If-Modified-Since header created here is not in GMT, but in local time zone.

According to https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since HTTP dates are always expressed in GMT, never in local time.

Objective

1. Please convert it to UTC, using .toUTCString() applied to that date.
2. Please make sure to add a test that verifies that If-Modified-Since is expressed in the GMT time zone

[justadreamer]

fixed as part of #113