properly handle embedded (anonymous) fields

[faryon93]

The json package handles marshaling of an anonymous field as following:

type Sub struct {
	Foo string `json:"foo"`
}

type Top struct {
	*Sub
	Bar string `json:"bar"`
}

func main() {
	v := Top{
		&Sub{"Hello"},
		"World!",
	}
	
	buf, _ := json.Marshal(v)
	fmt.Println(string(buf))
}

{"foo":"Hello","bar":"World!"}

With sheriff the anonymous field would be ignored. To make the output of sheriff as close as possible to the embedded json package, this pull request contains the changes to reflect the desired behaviour.

Kind regards,
Maxi

[coveralls]

Coverage increased (+1.4%) to 81.707% when pulling d0d9181 on faryon93:master into 2245bc6 on liip:master.

[mweibel]

Awesome, thanks!