http-client: Backport Guile fix for 'read-chunk-header'.

Fixes the wrong-type-arg exception initially reported
at <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19976#5>
by Ricardo Wurmus <rekado@elephly.net>.

* guix/http-client.scm (read-chunk-header): Backport Guile commit 53b8d5f.