fix missing collection hostname

[ghost]

GetCollections and GetPublishableCollections now take a hostname
parameter to allow setting the collecion hostname.

All collections used in memory now have their hostname set.

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• I have signed the CLA
  closes Collection.hostName is empty in the logs #161

[ghost]

This does seem like a lot of extra work compared to storing the hostname in the collection. The post could then get it's hostname from it's associated collection and we wouldn't be passing it around.

Then for things like export, we could omit the hostname value to avoid issues moving blogs across deployments.

But that is for another day, this fixes the mentioned issue.

[thebaer]

Yeah, I'm just not excited about adding a variable to the Collection struct that isn't going to change between different Collections. This seems like the least-worst way to do things, at least.

But looks good, thanks!