Extension.get API - Only apply special filters once (key/keys/full_name)

Filtering extensions by key is a good idea -- so good we have three ways to do it! ;)

All of these fields accept a `string|array<string>` input, which is different from
the typical filtering supported by `_civicrm_api3_basic_array_get()` (which only
matches on values)...  If you happen to choose a basic filter that passes both
filters (i.e.  a simple string-match), then it works.  But if you choose a filter
where they behave differnetly (i.e.  an array), then you get weird results.

api/v3/Extension.php

@@ -368,7 +368,13 @@ function civicrm_api3_extension_get($params) {
       $result[] = $info;
     }
   }
-  $filterableFields = array('id', 'key', 'type', 'status', 'path');
+
+  // These fields have been filtered already, and they have special semantics.
+  unset($params['key']);
+  unset($params['keys']);
+  unset($params['full_name']);
+
+  $filterableFields = array('id', 'type', 'status', 'path');
   return _civicrm_api3_basic_array_get('Extension', $params, $result, 'id', $filterableFields);
 }