Query constraint on other query throws "improper usage of $inQuery"

Hi,

Constraints based on results of other query doesn't seem to work in parse-server.
Simple iOS example (working with Parse.com):

```
PFQuery *fooQuery = [FooObject query];

PFQuery *barQuery = [BarObject query];
[barQuery whereKey:@"innerFooObject" matchesQuery:fooQuery];
```

Throws 

```
"Error Domain=Parse Code=102 \"improper usage of $inQuery\"
```
