Select tag nil issue

When a `select_tag` helper is used
And the `include_blank` option is not set
Then the `content_tag` method raised an error

By fixing the issue in Pedrino's `form_helpers.rb`, the method works as expected.

The prompt, or `include_blank` option should only be included, if `include_blank` is set.

This fixes #1092, but I think it would be best to get this fixed with Padrino directly, and then update the source in this repo with the updated fix.

Curious, why are you vending Padrino locally?