Support specifying locals with assert_template in view tests

I'd like to be able to specify the data that is passed to a partial by the template calling render. For example:

# in widgets index test
@widgets = [widget = mock]
render "widgets/index.html.erb"
assert_template :partial => "widgets/_widget", :locals => { :widget => widget }

# in widgets/index.html.erb
<% @widgets.each do |widget| -%>
  <%= render :partial => "widget", :locals => { :widget => widget } %>
<% end -%>

Patch forthcoming