使用图片Base64，约160KB，进行测试。
对比库：
- fastjson
- stdjson
速度对比：
fastjson
运行：
go test -benchmem -bench=BenchmarkImageBase64_fastjson$结果：
goos: darwin
goarch: amd64
pkg: github.com/xpzouying/learning_golang/json_marshal_benchmark
BenchmarkImageBase64_fastjson-8 20000 65925 ns/op 294927 B/op 1 allocs/op
PASS
ok github.com/xpzouying/learning_golang/json_marshal_benchmark 2.077s
stdjson
运行：
go test -benchmem -bench=BenchmarkImageBase64_stdjson$结果：
goos: darwin
goarch: amd64
pkg: github.com/xpzouying/learning_golang/json_marshal_benchmark
BenchmarkImageBase64_stdjson-8 500 3267756 ns/op 221387 B/op 4 allocs/op
PASS
ok github.com/xpzouying/learning_golang/json_marshal_benchmark 2.515s