luci-base: use actual JSON-RPC for verifying ubus RPC URL

Sending GET request to the main RPC base URL and expecting HTTP response
code 400 had two flaws:
1. It was not verifying actual JSON-RPC interface availability
2. It did not allow implementing support for new requests

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Jo-Philipp Wich <jo@mein.io>