fix(region_config): remove mutation of global signatureVersion

Author: kuhe

lib/region_config.js

@@ -64,11 +64,18 @@ function configureEndpoint(service) {
 
       // signature version
       if (!config.signatureVersion) {
-        config.signatureVersion = (service.api && service.api.signatureVersion) || 'v4';
+        // Note: config is a global object and should not be mutated here.
+        // However, we are retaining this line for backwards compatibility.
+        // The non-v4 signatureVersion will be set in a copied object below.
+        config.signatureVersion = 'v4';
       }
 
       // merge config
-      applyConfig(service, config);
+      applyConfig(service, Object.assign(
+        {},
+        config,
+        { signatureVersion: (service.api && service.api.signatureVersion) || 'v4' }
+      ));
       return;
     }
   }