fix the examples, so they run as tests

[kpacha]

... and cover the Unmarshal function too

[phamhongviet]

Isn't it already done in https://github.com/phamhongviet/godsv/blob/master/godsv_test.go#L9 ?
May be I should put simple examples for Marshal and Unmarshal in readme.

[kpacha]

This way, you are also black-box testing the lib, because these code is outside the godsv package, so it has the same visibility clients have.