Add ability to omit params member from request (#62)

The JSON-RPC 2.0 specification allows the params member of a request to
be omitted [1]. Before this commit, this library did not allow the
params member to be omitted when sending a request. When the params
argument of the Conn.Call/Conn.DispatchCall or Conn.Notify method was
set to nil, then Request.Params was set to the JSON encoding of nil
which is null.

This commit adds a CallOption named OmitNilParams. If OmitNilParams is
used when sending a request with params set to nil, then the params
member in the JSON encoding of Request is omitted. If the OmitNilParams
option is not used then the previous behavior is maintained. In other
words, the changes in this commit are backwards compatible.

References
[1]: https://www.jsonrpc.org/specification#request_object

call_opt.go

@@ -46,3 +46,12 @@ func StringID() CallOption {
 		return nil
 	})
 }
+
+// OmitNilParams returns a call option that instructs requests to omit params
+// values of nil instead of JSON encoding them to null.
+func OmitNilParams() CallOption {
+	return callOptionFunc(func(r *Request) error {
+		r.OmitNilParams = true
+		return nil
+	})
+}