luci-lib-base: forward luci.http.context.request.message to ucode

Some existing LuCI application code accesses the
`luci.http.context.request.message.params` HTTP parameter table directly.

Forward the `luci.http.context.request.message` object to the ucode
`luci.http.message` instance in order to properly support this.

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