Fix keep action of build artifacts widget

app/views/projects/builds/_sidebar.html.haml

@@ -27,7 +27,7 @@
       - if @build.artifacts?
         .btn-group.btn-group-justified{ role: :group }
           - if @build.artifacts_expire_at
-            = link_to keep_namespace_project_build_artifacts_path(@project.namespace, @project, @build), class: 'btn btn-sm btn-default' do
+            = link_to keep_namespace_project_build_artifacts_path(@project.namespace, @project, @build), class: 'btn btn-sm btn-default', method: :post do
               Keep
 
           = link_to download_namespace_project_build_artifacts_path(@project.namespace, @project, @build), class: 'btn btn-sm btn-default' do