Use MarshalJSON directly on notification

[moredure]

Actually on nil notification it panics before as well, so maybe adding nil check to MarshalJSON or on other function may be good, what do you think?

[coveralls]

Coverage remained the same at 100.0% when pulling 0094dbad6fcddb57f44732fbd0afeb1183decb49 on moredure:patch-7 into a09d4b5ea2a812939e148b54abb9c2a517aa2411 on sideshow:master.

[moredure]

package main_test

import (
	"encoding/json"
	"testing"
)

type S struct {
	X []byte `json:"x"`
}

func (s *S) MarshalJSON() ([]byte, error) {
	return s.X, nil
}

func BenchmarkRun(b *testing.B) {
	s := &S{
		X: []byte("1"),
	}
	b.Run("one", func(b *testing.B) {
		b.ReportAllocs()
		for i := 0; i < b.N; i++ {
			json.Marshal(s)
		}
	})
	b.Run("one", func(b *testing.B) {
		b.ReportAllocs()
		for i := 0; i < b.N; i++ {
			s.MarshalJSON()
		}
	})
}
BenchmarkRun/one
BenchmarkRun/one-4         	 5523985	       197.8 ns/op	       8 B/op	       1 allocs/op
BenchmarkRun/one#01
BenchmarkRun/one#01-4      	1000000000	         0.3392 ns/op	       0 B/op	       0 allocs/op

At least one additional allocation per payload (leaking param)