pass through params if no mapping is specified

[kevin--]

What

• if the client does not specify a mapping when calling Add, pass through the params object

Why

• I was recently working on implementing an Model Context Protocol server with this library, and it is impossible to specify optional named parameters.
• Some Clients send the _meta parameter and some do not, so JSON RPC breaks in all these situations.
• This way the implementor can check for parameters and determine what to do