Change message params types to match JSON-RPC spec

Since the interface is supposed to describe message format to match [JSON-RPC spec](http://www.jsonrpc.org/specification),
the params type must be specified as either `Array` or `Object`.

From spec:
> 4.2 Parameter Structures
>
> If present, parameters for the rpc call MUST be provided as a Structured value. Either by-position through an Array or by-name through an Object.

specification.md

@@ -89,7 +89,7 @@ interface RequestMessage extends Message {
 	/**
 	 * The method's params.
 	 */
-	params?: any
+	params?: Array<T> | object;
 }
 ```
 
@@ -164,7 +164,7 @@ interface NotificationMessage extends Message {
 	/**
 	 * The notification's params.
 	 */
-	params?: any
+	params?: Array<T> | object;
 }
 ```