Fix bug in relationships API introduced by #6482 (#6527)

It was merge when it needed to be deep_merge. And added some tests

app/controllers/api/v1/accounts/relationships_controller.rb

@@ -21,6 +21,6 @@ class Api::V1::Accounts::RelationshipsController < Api::BaseController
   end
 
   def account_ids
-    @_account_ids ||= Array(params[:id]).map(&:to_i)
+    Array(params[:id]).map(&:to_i)
   end
 end

app/presenters/account_relationships_presenter.rb

@@ -45,7 +45,7 @@ class AccountRelationshipsPresenter
       maps_for_account = Rails.cache.read("relationship:#{@current_account_id}:#{account_id}")
 
       if maps_for_account.is_a?(Hash)
-        @cached.merge!(maps_for_account)
+        @cached.deep_merge!(maps_for_account)
       else
         @uncached_account_ids << account_id
       end