Fix error when default branch tries to be autosaved

For some reason when branch from V2 models is being saved, it tries to
also update relationships for V3 models, at least in tests. This fails,
because default_branch association on the V3 Repository model has a
primary key set to [:id, :default_branch]. In theory we use composite
keys plugin, but it seems that it doesn't cover that case and because of
that AR fails with an error "[:id, :default_branch] is not a Symbol"
when it tries to call Model#send with primary_key as an argument.

This commit fixes the issue by overriding the send method on the
Repository model to not fail when AR does
repository.send([:id, :default_branch_name]), when it needs to fetch
a primary key.

This is hacky, but I haven't found a nicer way to get around it.

lib/travis/api/v3/models/repository.rb

@@ -49,5 +49,17 @@ module Travis::API::V3
     rescue ActiveRecord::RecordNotUnique
       branches.where(name: name).first
     end
+
+    def id_default_branch
+      [id, default_branch_name]
+    end
+
+    def send(name, *args, &block)
+      if name == [:id, :default_branch]
+        name = :id_default_branch
+      end
+
+      __send__(name, *args, &block)
+    end
   end
 end