Allow a Liquid::Drop to be passed into Template#render

Right now, if you pass a `Liquid::Drop` to `Template#render`, it barfs saying it wants a `Liquid::Context` or a `Hash`.

This change lets you pass in a `Liquid::Drop` as well. You can already do this manually by just making a `Liquid::Context` with the drop as the first argument, but this lets the `#render` method take care of that for you.