Added token model unit test
Added token model unit test

Asserts that `expires_in` is calculated if `accessTokenExpiresAt` is
provided.

maxtruxa/node-oauth2-server,visvk/node-oauth2-server,thomseddon/node-oauth2-server,mjsalinger/node-oauth2-server,razvanz/node-oauth2-server,manjeshpv/node-oauth2-server,oauthjs/node-oauth2-server