[#2279] Populated the Timesheet filter with the values from the Billable field

using the OverheadHelper.

app/views/timesheet/_overhead_form.html.erb

@@ -3,6 +3,8 @@
     <%= l(:overhead_field_billing_status) %>
   </label>
   <br />
-  <%= select_tag('timesheet[billable][]','', {:multiple => true, :size => list_size}) %>
+  <%= select_tag('timesheet[billable][]',
+                 options_for_select(billable_values),
+                 {:multiple => true, :size => list_size}) %>
 
 </p>