generated Last-modified header value uses the wrong timezone

[avdv]

The StaticFileHandler generates Last-Modified headers with the timezone set to the current system's timezone.

It should use GMT as defined by http://tools.ietf.org/html/rfc2616#section-3.3

HTTP/1.1 200 OK
last-modified: Thu, 18 Dec 2014 11:21:17 CET
...

REDbot complains again:

• The last-modified header's value isn't a valid date.

  HTTP dates have very specific syntax, and sending an invalid date can
  cause a number of problems, especially around caching. Common problems
  include sending "1 May" instead of "01 May" (the month is a fixed-
  width field), and sending a date in a timezone other than GMT. See the
  HTTP specification for more information.