Specify the full path to the NEWLINE constant

I was getting the following error when trying to use the `select_tag` helper:

```
NameError: uninitialized constant Middleman::CoreExtensions::DefaultHelpers::NEWLINE
	/Users/matt/.rvm/gems/ruby-2.0.0-p353/gems/middleman-core-3.2.1/lib/middleman-more/core_extensions/default_helpers.rb:52:in `block in content_tag'
	/Users/matt/.rvm/gems/ruby-2.0.0-p353/gems/middleman-core-3.2.1/lib/middleman-more/core_extensions/default_helpers.rb:52:in `each'
	/Users/matt/.rvm/gems/ruby-2.0.0-p353/gems/middleman-core-3.2.1/lib/middleman-more/core_extensions/default_helpers.rb:52:in `content_tag'
	/Users/matt/.rvm/gems/ruby-2.0.0-p353/gems/middleman-core-3.2.1/lib/vendored-middleman-deps/padrino-helpers-0.11.4/lib/padrino-helpers/tag_helpers.rb:146:in `safe_content_tag'
	/Users/matt/.rvm/gems/ruby-2.0.0-p353/gems/middleman-core-3.2.1/lib/vendored-middleman-deps/padrino-helpers-0.11.4/lib/padrino-helpers/form_helpers.rb:626:in `select_tag'
...truncated
```

After investigating it, I created the included feature to test this, and fixed the code by specifying `::Padrino::Helpers::TagHelpers::NEWLINE`, assuming that is the `NEWLINE` constant that you wanted to use in this method.

Let me know if there is anything else I can do to get this in, or if I missed something here.