Return password form if content requires password

Mirroring the [default WordPress `get_the_content` behavior](https://core.trac.wordpress.org/browser/tags/3.9.1/src/wp-includes/post-template.php#L216), Timber's `get_content` and `content` should return the password form for unauthenticated users. There is more discussion about how this is currently handled in #217. I'd argue that it would be ideal for the developer to not concern themselves with conditionally handling password authentication in each template.

I had some issues getting VVV running in an attempt to verify the included unit test, so please verify on my behalf.