Convert property_type validation from enum to string

Zoopla documentation allows sending any string as property_type as per https://realtime-listings.webservices.zpg.co.uk/docs/latest/documentation.html#appendix_attributes_property_type

src/Request/SendProperty.php

@@ -60,7 +60,7 @@ class SendProperty extends RequestBase
 		'total_bedrooms' => array('type' => 'integer', ),
 
 
-		'property_type' => array('type' => 'enum', 'class' => PropertyType::class, ),
+		'property_type' => array('type' => 'string', ),
 		'listed_building_grade' => array('type' => 'enum', 'class' => ListedBuildingGrade::class, ),
 		'tenure' => array('type' => 'enum', 'class' => Tenure::class, ),
 		'central_heating' => array('type' => 'enum', 'class' => CentralHeating::class, ),
@@ -122,4 +122,4 @@ class SendProperty extends RequestBase
 		'waterfront' => array('type' => 'boolean', ),
 		'wood_floors' => array('type' => 'boolean', ),
 	);
-}
+}