luci-base: dispatcher.uc: only flush HTTP headers after rendering output

Ensure to first completely render the action function before flushing HTTP
headers since the invoked action logic might modify the HTTP headers itself.

Fixes: e7afd0d3 ("luci-base: fix luci.http.close()")
Ref: https://github.com/openwrt/luci/commit/e7afd0d3

#commitcomment-88736854
Signed-off-by: Jo-Philipp Wich <jo@mein.io>