fix(service): use service signatureVersion as default (#4284)

Author: kuhe

lib/region_config.js

@@ -63,7 +63,9 @@ function configureEndpoint(service) {
       }
 
       // signature version
-      if (!config.signatureVersion) config.signatureVersion = 'v4';
+      if (!config.signatureVersion) {
+        config.signatureVersion = (service.api && service.api.signatureVersion) || 'v4';
+      }
 
       // merge config
       applyConfig(service, config);