When we do a #fetch on Mogli::User, we should get association relations (i.e., location)

If you do:

u1 = Mogli::User.find(fb_uid, mogli_client)
u1.location # Gets filled
-> bla bla

But if you do:
u2 = Mogli::User.new({id: fb_uid}, mogli_client)
u2.fetch.location
-> nil

This patch fixes this behaviour
Merge pull request #132 from gaizka/when-fetching-copy-associations-attributes

When we do a #fetch on Mogli::User, we should get association relations ...