Previously, JSON-RPC clients accessed URI "/", and the JSON-RPC server did not care about the URI at all, and would accept any URI as valid.
Change the JSON-RPC server to require URI "/" for all current accesses.
This changes enables the addition of future interfaces at different URIs, such as pull request #1982 which demonstrates HTTP REST wallet download.
Or, a future, breaking change in JSON-RPC interface could be introduced by serving JSON-RPC calls from new URI "/v2/".