set 'alt' attribute for image_submit_tag
Bump rack dependency to 1.4.3

It includes security bug fixes and changes the initialization of
Rack::File to accept a hash, otherwise generating warnings.

See 295806e for the warnings fix.
Never treat action or controller as unpermitted params
Revert "unpermitted params" exception -- it's just not going to work. See the discussion on https://github.com/rails/strong_parameters/pull/75.
Merge pull request #8810 from NARKOZ/image-submit-tag

set 'alt' attribute for image_submit_tag