When using a render :partial with :layout call, without giving a block,
if the given :partial had another render :partial call, the layout was
not being rendered. This commit fixes this context by storing variables
before rendering the partial, so they are not overrided in any successive
call to render partials down the path. All ActionPack tests are ok.

Signed-off-by: José Valim <jose.valim@gmail.com>