Handle 'If-Modified-Since' header

[Luukdegram]

Clients can send the header If-Modified-Since to save bandwidth. Although clients can ignore this, servers must comply with this header.

This means we need to be able to parse the following three formats (due to earlier HTTP versions): If-Modified-Since: Fri, 31 Dec 1999 23:59:59 GMT If-Modified-Since: Friday, 31-Dec-99 23:59:59 GMT If-Modified-Since: Fri Dec 31 23:59:59 1999