[Multiple issue asignees] fix importers and specs for them

lib/gitlab/fogbugz_import/importer.rb

@@ -127,7 +127,7 @@ module Gitlab
             title:        bug['sTitle'],
             description:  body,
             author_id:    author_id,
-            assignee_id: assignee_id,
+            assignee_ids: [assignee_id],
             state:        bug['fOpen'] == 'true' ? 'opened' : 'closed',
             created_at:   date,
             updated_at:   DateTime.parse(bug['dtLastUpdated'])

lib/gitlab/github_import/issue_formatter.rb

@@ -10,7 +10,7 @@ module Gitlab
           description: description,
           state: state,
           author_id: author_id,
-          assignee_id: assignee_id,
+          assignee_ids: [assignee_id],
           created_at: raw_data.created_at,
           updated_at: raw_data.updated_at
         }

lib/gitlab/google_code_import/importer.rb

@@ -97,7 +97,7 @@ module Gitlab
             title:        raw_issue['title'],
             description:  body,
             author_id:    project.creator_id,
-            assignee_id: assignee_id,
+            assignee_ids: [assignee_id],
             state:        raw_issue['state'] == 'closed' ? 'closed' : 'opened'
           )
 

spec/lib/gitlab/google_code_import/importer_spec.rb

@@ -49,7 +49,7 @@ describe Gitlab::GoogleCodeImport::Importer, lib: true do
       expect(issue).not_to be_nil
       expect(issue.iid).to eq(169)
       expect(issue.author).to eq(project.creator)
-      expect(issue.assignee).to eq(mapped_user)
+      expect(issue.assignees).to eq([mapped_user])
       expect(issue.state).to eq("closed")
       expect(issue.label_names).to include("Priority: Medium")
       expect(issue.label_names).to include("Status: Fixed")