Hello,

I encountered a bug in the HTTP header parsing bug when trying to
download a file via Guix.  The response had a Content-Type header, but
with no value, like so:

    Content-Type: 

From reading the W3C spec[0], an unknown Content-Type header can be
treated as if it were an application/octet-stream type.  I'm unsure if
that means even Content-Type values that have invalid syntax should be
accepted or not, so I didn't try to handle them.  I'm not even sure if
we should be translating the empty string to application/octet-stream.
I know that it shouldn't throw an excpetion, but maybe it should just
return the empty list and the user can decide how to interpret it?

Anyway, here's a patch to get the ball rolling.