Always call 'to_liquid' on stuff in map filter and allow to_liquid to be...

People are doing this

```{{cart.items|map:'to_liquid'|map:'product'|map:'to_liquid'|map:'id'|join:','}}```

which is broken since #230.

For backwards compatibility, this PR allows to call "to_liquid" on drops. Also, automatically call "to_liquid" on all the things if possible before doing anything.

Please review @dylanahsmith