Tighter request handling

[mattias-p]

In the RPCAPI reference there is a "Request Handling" section that lists a number of quirks in how we interpret JSON-RPC requests. We should restrict the set of inputs so that users must supply a string when a string is expected, a boolean when a boolean is expected and a number when a number is expected.

I don't think we ever intended to implement these quirks. It's just something that happened along the way. Unfortunately, since we're promising these behaviors in our documentation, removing them will be a breaking change.