express-hbs: Subexpressions don't work as expected

This is a follow up on #55 but might not directly be related.

Using sub-expressions like {{encode (url)}} (as in https://github.com/halfdan/hbs-weird/) where the url helper returns a URL and encode executes encodeURIComponent the expression is replaced by only the result of the url helper. Using {{encode "http://foo.bar/?baz=muh"}} on its own seems to work just fine.

Example case in https://github.com/halfdan/hbs-weird/