STAC: fix collection search (#1073)

Author: tomkralidis

pycsw/ogc/api/records.py

@@ -595,7 +595,7 @@ def items(self, headers_, json_post_data, args, collection='metadata:main'):
                     ids = ','.join(f'"{x}"' for x in v.split(','))
                     query_args.append(f"identifier IN ({ids})")
                 elif k == 'collections':
-                    if isinstance(collections, str):
+                    if isinstance(v, str):
                         collections = ','.join(f'"{x}"' for x in v.split(','))
                     else:
                         collections = ','.join(f'"{x}"' for x in v)