Support both integer and string as epic count key

Epic state uses enum for saving state (other issuables use
string), because of this inconsistency we override `count_key`
which should return state as string converted to symbol
(.e.g. :closed). In rails 5 these integer states are automatically
converted to strings by Active Record.

But if subquery is used (to optimize searching), Active Record doesn't
convert state into string automatically (probably it can not deal with
the more complex query).

This means that epic state can be either integer (if subquery is used)
or string (if AR converted state to string automatically), so in
`count_key` we should convert state into string only if it's an integer.

ee/app/finders/epics_finder.rb

@@ -55,7 +55,13 @@ class EpicsFinder < IssuableFinder
   private
 
   def count_key(value)
-    Array(value).last.to_sym
+    last_value = Array(value).last
+
+    if last_value.is_a?(Integer)
+      Epic.states.invert[last_value].to_sym
+    else
+      last_value.to_sym
+    end
   end
 
   # rubocop: disable CodeReuse/ActiveRecord

ee/spec/finders/epics_finder_spec.rb

@@ -202,5 +202,22 @@ describe EpicsFinder do
 
       expect(results).to eq('opened' => 2, 'closed' => 1, 'all' => 3)
     end
+
+    context 'when using group cte for search' do
+      before do
+        stub_feature_flags(use_subquery_for_group_issues_search: false)
+      end
+
+      it 'returns correct counts when search string is used' do
+        results = described_class.new(
+          search_user,
+          group_id: group.id,
+          search: 'awesome',
+          attempt_group_search_optimizations: true
+        ).count_by_state
+
+        expect(results).to eq('opened' => 1, 'closed' => 1, 'all' => 2)
+      end
+    end
   end
 end