Restore adding items when no initial items

This got broken in bf6a99db1898af2f70f82c147dd5f2bfc3057d2c.
Fix #926 "xxf:get-request-header('User-Agent') returning an empty sequence"
Improve HTTP headers capitalization

This is related to #891 and #967.
Fix #891 "Proxy portlet header forwarding must not change header case"