Fix non-spec compliant Last-Modified header in response

I found that the date in the `Last-Modified` header of `.file()` responses are invalid according to http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.29 & http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3.1.

This patch should fix this issue, generating valid dates, along with an extra test in case of future regressions.