fixed condition on cache expiration

[mpatafio]

The condition on the expiration is wrong, it doesn't work as described in the documentation.
given a source.expiration = undefined

!!!source.expiration -> true
Boolean(source.expiration) -> false

[wonday]

@mpatafio You are right, thank you.

[wonday]

@mpatafio this PR still have problem.
if you set an expiration, it will reload every time. do not use cache.
I had modified again and commit to master.