Feature/673 xauth token

### Description
X-Auth-Token forwarded in notifications

Implements #673 
I understand that if X-Auth-token is not included in the HTTP request, then "" is uses as default, so nothing breaks (the code in this line will deal with that case https://github.com/telefonicaid/fiware-orion/pull/682/files#diff-25f406998622ce6403fd56a8ed9d0b05R287)

Why we use all lowercase in other cases  (e.g. fiware-service) and not "x-auth-token" in this place. To be coherent, I think we should do that.