read-header returns EOF at end, update (web http) docs

read-header returns EOF at end, update (web http) docs

* doc/ref/web.texi (HTTP): Add an example for declaring a header, and
  adapt to read-header change.

* module/web/http.scm (read-header): Return EOF for both values if there
  are no more headers, instead of #f.
  (read-headers): Adapt.