Add :as option to form_for helper

:as option can now be used to override default object name when building
form.

This is especially useful if you are using proxy object and don't want
world to know about it.
Should not try to focus on error input unless there is one.
Merge pull request #1479 from iwakura/master

Fix for autofocus in admin panel.
Now properly handles boolean values for app.cascade

cascade only supplied statuses or default ones
router cascading extra fail test (cascade => false or true  scenario)
modified test to assert that :ok statuses by default are not cascaded
one more test on cascade driven behavior
Fix incorrect namespace of #named_routes.
Disambiguate the behavior of `url`.
Merge pull request #1481 from padrino/fix-named-routes

Fix incorrect namespace of #named_routes. Fixes #1095
Merge pull request #1480 from Mingoville/router_cascade_boolean_fix

Router cascade boolean fix
Merge pull request #1477 from graudeejs/form_for_as

Add :as option to form_for helper