Check the actual status code for 204 and 304

The header is the full http header like: HTTP/1.1 304 Not Modified
So comparing this to an int always yields false
This also makes the 304 RFC compliant as the resulting content length
should otherwise be the length of the message and not 0.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>
Signed-off-by: Morris Jobke <hey@morrisjobke.de>