Mocking or Stubbing mtime for File::Stat - file

Question

My object GETs a file over HTTP.
It does so, using the If-Modified-Since header. If the files has not been modified since the time in the header a Not Modified response will be returned and the file should not be fetched&written. Like so:
class YouTube
#...
def parse
uri = URI("http://i.ytimg.com/vi/#{#id}/0.jpg")
req = Net::HTTP::Get.new(uri.request_uri)
if File.exists? thumbname
stat = File.stat thumbname
req['If-Modified-Since'] = stat.mtime.rfc2822
end
res = Net::HTTP.start(uri.hostname, uri.port) {|http|
http.request(req)
}
File.open(thumbname, 'wb') do |f|
f.write res.body
end if res.is_a?(Net::HTTPSuccess)
end
#...
end
I want to test both cases (in both cases, a file on disk exists). To do so, I'd need to stub either something in Net::HTTP, or I need to stub the File.stat to return an mtime for which I a sure the online resource will return a new or Not-modified-since.
Should I stub (or even mock) Net::HTTP? And if so, what?
Or should I stub mtime to return a date far in the past or far in the future to enforce or suppress the Not-modified Header?
Edit: Diving deeper into the matter, I learned that the i.ytimg.com-domain does not support these headers. So i'll need to solve this by inspecing JSON from the YouTube API. However, the problem "What and how to mock when testing if-modified-since-headers" still stands.
Here is how I conclude the domain does not support this:
$curl -I --header 'If-Modified-Since: Sun, 24 Mar 2013 17:33:29 +0100' -L http://i.ytimg.com/vi/D80QdsFWdcQ/0.jpg
HTTP/1.1 200 OK
Content-Type: image/jpeg
Date: Sun, 24 Mar 2013 16:31:10 GMT
Expires: Sun, 24 Mar 2013 22:31:10 GMT
X-Content-Type-Options: nosniff
Server: sffe
Content-Length: 13343
X-XSS-Protection: 1; mode=block
Cache-Control: public, max-age=21600
Age: 822
There is no "Last-Modified" there. Illustrated with another call, to exaple.com, which does support the if-modified-since headers.
$curl -I --header 'If-Modified-Since: Sun, 24 Mar 2013 17:33:29 +0100' -L example.com
HTTP/1.0 302 Found
Location: http://www.iana.org/domains/example/
Server: BigIP
Connection: Keep-Alive
Content-Length: 0
HTTP/1.1 302 FOUND
Date: Sun, 24 Mar 2013 16:47:47 GMT
Server: Apache/2.2.3 (CentOS)
Location: http://www.iana.org/domains/example
Connection: close
Content-Type: text/html; charset=utf-8
HTTP/1.1 304 NOT MODIFIED
Date: Sun, 24 Mar 2013 16:47:47 GMT
Server: Apache/2.2.3 (CentOS)
Connection: close