Non-persisted records try to make HTTP calls for associations with nil foreign_key

[valo]

If you create a new instance of a Her model and it has a belongs_to association which is nil it will try to fetch a record with ID=nil.

A short example:

user = User.new(name: "Karlin", organization_id: nil)
user.organization # this throws an exception as it tries to fetch GET /organizations

The error from the test in the PR:

  1) Her::Model::Associations handling associations without details returns nil if the foreign key is nil
     Failure/Error: @user_without_organization_and_not_persisted.organization.should be_nil
     Faraday::Adapter::Test::Stubs::NotFound:
       no stubbed request for get /organizations
     # ./lib/her/api.rb:94:in `request'
     # ./lib/her/model/http.rb:56:in `request'
     # ./lib/her/model/http.rb:81:in `get_raw'
     # ./lib/her/model/http.rb:70:in `get'
     # ./lib/her/model/associations/belongs_to_association.rb:84:in `fetch'
     # ./lib/her/model/associations/association_proxy.rb:11:in `nil?'
     # ./lib/her/model/associations/association_proxy.rb:40:in `method_missing'
     # ./spec/model/associations_spec.rb:215:in `block (3 levels) in <top (required)>'

I think the correct behavior is to return nil no matter if the record is persisted or not.

[hubert]

@valo could you rebase with what's in master so we can run the full travisci checks. once that's done, we're G2G.

[valo]

Done

[hubert]

thanks for contributing!