toString should handle `*string`

[breml]

For an argument of type *string, there is no case defined for the toString function, which causes the default case to be selected. The default case uses fmt.Sprintf("%v", v), which returns the address of the pointer. In my opinion, this is almost never the thing, which a user of toString is expected to get in the case of a Go template.

Instead, the referenced value of the pointer should be returned.

Link to current implementation: https://github.com/Masterminds/sprig/blob/master/strings.go#L174

Suggested implementation:

func strval(v interface{}) string {
	switch v := v.(type) {
	case string:
		return v
	case *string:
		if v == nil {
			return ""
		}
		return *v
	case []byte:
		return string(v)
	case error:
		return v.Error()
	case fmt.Stringer:
		return v.String()
	default:
		return fmt.Sprintf("%v", v)
	}
}