Fix bug when using partials in two successive Sinatra requests.

Without the patch to `lib/tilt/jbuilder.rb`, the new test case in `spec/sinatra_integration_spec.rb` blows up because the subsequent call to `set_locals` voids `@_jbuilder_view_path`.

This patch fixes that problem, and provides a regression test to ensure it doesn't get reintroduced by mistake.