Latest version panics on interface{}

[davrux]

The following program will panic, because interface type in struct.
Older Versions supported it.

package main

import (
    "fmt"

    "github.com/fatih/structs"
)

type MyStruct struct {
    PublicKey interface{}
}

func main() {

    mystruct := MyStruct{PublicKey: []byte("test")}
    s := structs.Map(mystruct)
    fmt.Printf("s %+v\n", s)
}

Tried to structs.Map an x509 certificate from crypto/x509 and it panics.
Older Version, commit e5ca5fe, does not have this problem.
Panic trace shows structs.go:119, where the last commit was around.

[fatih]

Was fixed with #62