Fixed css class for filter field

Author: Nikolay Moskvin

app/views/rails_admin/main/index.html.haml

@@ -96,7 +96,7 @@
     .well
       %span#filters_box
       %hr.filters_box{style: "display:#{@ordered_filters.empty? ? 'none' : 'block'}"}
-      %input.input-small{name: "query", type: "search", value: query, placeholder: t("admin.misc.filter")}
+      %input.form-control.input-small{name: "query", type: "search", value: query, placeholder: t("admin.misc.filter")}
       %button.btn.btn-primary{type: "submit", :'data-disable-with' => "<i class='icon-white icon-refresh'></i> ".html_safe + t("admin.misc.refresh")}
         %i.icon-white.icon-refresh
         = t("admin.misc.refresh")