Remove unnecessary stringify_keys

A set of `stringify_keys` was introduced 9 years ago in dfac1cea
to fix helpers that would treat string and symbol keys differently
and possibly create duplicate entries.

Currently Rails requires versions of Ruby >= 1.9; this means that
Hashes preserve the keys in the order in which they were inserted.

For this reason, there is no need to stringify keys anymore. Even
if someone were to write code like:

```rhtml
<%= time_field_tag 'timestamp', 1200, "type" => "password" %>
```

then a simple `options.update` (without `stringify_keys`) would
generate the temporary hash `{"type"=>"password", type: :time}`
which, passed to `text_field_tag`, would result in:

```html
<input id="timestamp" name="timestamp" type="time" value="1200" />
```

since [`text_field_tag` already invokes `stringify_keys` on its options](https://github.com/rails/rails/blame/482e80e84440136763b47c0fba2ed65a7823f022/actionview/lib/action_view/helpers/form_tag_helper.rb#L182)
and `type: :time` is the last assigned option.
Pass :type options as symbol, not string

This commit is an optional addition to the previous commit cda0489.

The previous commit removed unnecessary `stringify_keys` from some
field helpers, and in doing that, replaced the types passed from
strings to symbols. For instance, the following is taken from cda0489:

```diff
- text_field_tag(name, value, options.stringify_keys.update("type" => "hidden"))
+ text_field_tag name, value, options.update(type: :hidden)
```

This commit replaces strings with symbols even in those field helpers
that were not affected by the previous commit.

Therefore, this commit has no immediate (performance) benefits on the code,
but in my opinion provides more coherence to the whole file, and leads the
way for the future versions of Ruby/Rails (GC symbols etc.).

If you prefer to keep the code as it is, fine by me! :)