Pass header to the request session object to enable authorization via header

[r-hans]

Hi all,

my use-case:

• the authorization details for my REST request are coming from "outside" as http-header
• I as developer, want to forward this header to the request session created by the package as native as possible

Basically, we need to add another argument "header" to AtlassianRestAPI and add it to the request session. That is, the only structural change.

I will raise a PR once I have time.

Greetings,
Raphael