title: REST API - GET Request filter by Ids
body: <p>Let's say I have an index endpoint to list units
  <code>GET /projects/1/units/?id[]=1,2,3</code></p>
  
  <p>and units with ids(1,2) belongs to project with id (1) but unit with id (3) doesn't belong to this project</p>
  
  <p>What should be the proper response for this case, please note it's <code>GET</code> collection endpoint.</p>
  
  <p>Response should be <code>200</code> with list of available units or <code>400</code> Bad Request because the unit with id=3 doesn't belong to the project?</p>
  
  <p>Thanks</p>