List file collaborations - Does not return a collaborations resource in response

## Description of the issue

See #149 for some context.

List file collaborations - 200 response is supposed to be a collaborations resource. But it seems this is incorrect.

It is returning this instead:
`
array(3) { 'next_marker' => string(0) "" 'previous_marker' => string(0) "" 'entries' => array(0) { } }
`

## Current content


## Expected content
It should probably be its own resource but I am unsure of how the Box team would want to handle this. Adding a new resource for a single endpoint seems not wise.