rpc/lib/server: add handlers tests

Follow up of PR https://github.com/tendermint/tendermint/pull/724
For https://github.com/tendermint/tendermint/issues/708

Reported initially in #708, this bug was reconfirmed
by the fuzzer.

This fix ensures that:
* if the user doesn't pass in `"id"` that we send them back
a message in an error telling them to send `"id"`. Previously
we let the handler return a 200 with nothing.
* passing in nil `params` doesn't crash
* not passing in `params` doesn't crash
* passing in non-JSON parseable data to `params` doesn't crash

rpc/lib/server/handlers.go

@@ -118,6 +118,8 @@ func makeJSONRPCHandler(funcMap map[string]*RPCFunc, logger log.Logger) http.Han
 		// The Server MUST NOT reply to a Notification, including those that are within a batch request.
 		if request.ID == "" {
 			logger.Debug("HTTPJSONRPC received a notification, skipping... (please send a non-empty ID if you want to call a method)")
+			// Not sending back a response here because according the JSONRPC
+			// specification Section 4.1, we SHOULD NOT one back when "id" == "".
 			return
 		}
 		if len(r.URL.Path) > 1 {