Remove duplicate stringify_keys in text_field_tag

All the methods that invoke `text_field_tag` (such as `hidden_field_tag`) do not need to call `stringify_keys` on their `options` parameter since the `text_field_tag` method [is already doing it internally](https://github.com/claudiob/rails/blob/4159134524f4c78d008eef9d9a17f73a3172dcc8/actionview/lib/action_view/helpers/form_tag_helper.rb#L182):

```ruby
def text_field_tag(name, value = nil, options = {})
  tag :input, { "type" => "text", "name" => name, "id" => sanitize_to_id(name), "value" => value }.update(options.stringify_keys)
end
```

[Note 1: My code uses `merge` to respect the existing behavior of duplicating the `options` hash before updating its keys, see https://github.com/rails/rails/pull/17096#issuecomment-57223827]

[Note 2: My code uses symbols instead of strings (e.g.: `:hidden`) to look forward to future version of Ruby/Raiks (GC symbols); the result of the method, however, is the same, because the symbols are stringified inside `text_field_tag`]

[Note 3: I had previously created a similar PR #17096 but decided to split it into multiple PRs given the feedback received in the comments]