Add Avatar to requestUser structure

[TE-tatuonagamatu]

This is pull request for resolving Issue: Setting avatar does not work

Avatar was added to User structure. But it is not in reqeustUser structure. Before posting /user API, User structure is converted to requestUser. Avatar is not defined in requestUser and not copied from User. So avatar setting is not work.

Avatar is added also for requestUser and copied from User.

[TE-tatuonagamatu]

Checked with following code

package main

import (
	"fmt"

	intercom "github.com/intercom/intercom-go"
)

func main() {
	ic := intercom.NewClient("access_token", "")

	user := intercom.User{
		UserID: "5750790484393984",
		Email:  "[email protected]",
		Name:   "Tatsuo Nagamatsu",
		Avatar: &intercom.UserAvatar{Type: "avatar", ImageURL: "https://avatars2.githubusercontent.com/u/116613?v=4"},
	}
	savedUser, err := ic.Users.Save(&user)
	if err != nil {
		panic(err)
	}

	fmt.Println(savedUser.Avatar.String())
}

[TE-tatuonagamatu]

CustomAttributes are missing in UserCompany that holds information about belonging company. Add this fix to this PR. I created another PR #116 for CustomAttributes issue