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>

libs/luci-lib-base/luasrc/http.lua

@@ -140,5 +140,6 @@ context = {
 		content        = function(self, ...) return content(...)        end;
 		getcookie      = function(self, ...) return getcookie(...)      end;
 		setfilehandler = function(self, ...) return setfilehandler(...) end;
+		message        = L.http.message
 	}
 }