Added tests to render helper that expect `render partial: @foo` to
automatically call @foo.to_partial_path

Calling `render @foo` allows local variables but not options to be
passed to the partial renderer. The correct way to render an object AND
pass options to the partial renderer is to pass the object in the
`:partial` parameter. However, there were previously no tests for this
behaviour (in `render_helper_test.rb` at least).