fix flash_tag() with multiple attributes.

flash_tag() with multiple attributes raises NoMethodError when some attributes are not given.

When flash is set in controller:

    flash[:error] = 'error message'
    flash[:success] = 'success message'

and use flash_tag() in view:

    flash_tag(:error, :warning, :success)

Then raise error:

    NoMethodError: undefined method `<<' for nil:NilClass