fix uninitialized constant Tilt::HandlebarsTemplate::Pathname

using a partial that has not already been registered raising the "uninitialized constant Tilt::HandlebarsTemplate::Pathname" error.

```ruby
require 'tilt/handlebars'
tmpl = Tilt.new 'hello.hbs'
puts tmpl.render name: 'railsconf'
```

templates:

__hello.hbs__ template:
```handlebars
{{> header}}

hello {{name}}!
```

__header.hbs__ partial:
```handlebars
<h1>HEADING</h1>
```