Fixes #3195 (#3201)

* Adds error reproduction

* Fix transform in order to accept nested array results in $in/$nin

The error originated by the nesting of 2 array in $in [["..."]], using _.flatMap with those will guarantee at the lower level
that the query is properly resolved

* nits

* _.flatMap the $in/$nin values

src/Adapters/Storage/Postgres/PostgresStorageAdapter.js

@@ -1,5 +1,6 @@
 import { createClient } from './PostgresClient';
 import Parse            from 'parse/node';
+import _                from 'lodash';
 
 const PostgresRelationDoesNotExistError = '42P01';
 const PostgresDuplicateRelationError = '42P07';
@@ -296,10 +297,10 @@ const buildWhereClause = ({ schema, query, index }) => {
         }
       }
       if (fieldValue.$in) {
-        createConstraint(fieldValue.$in, false);
+        createConstraint(_.flatMap(fieldValue.$in, elt => elt), false);
       }
       if (fieldValue.$nin) {
-        createConstraint(fieldValue.$nin, true);
+        createConstraint(_.flatMap(fieldValue.$nin, elt => elt), true);
       }
     }