Added Country to searchOwned

The documentation for this function points to there being a country argument (https://developer.nexmo.com/api/numbers#getOwnedNumbers), but the function doesn't allow this.

Adding the string here allows this to be passed through.

Author: footballencarta

src/Numbers/Client.php

@@ -239,6 +239,7 @@ public function searchOwned($number = null, array $options = []) : array
 
         // These are all optional parameters
         $possibleParameters = [
+            'country' => 'string',
             'pattern' => 'string',
             'search_pattern' => 'integer',
             'size' => 'integer',