Fix bug when creating repo with non-default visibility

Author: jpverdejo

pom.xml

@@ -2,7 +2,7 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>org.kohsuke</groupId>
   <artifactId>cortexapps-github-api</artifactId>
-  <version>1.327</version>
+  <version>1.328</version>
   <name>GitHub API for Java</name>
   <url>https://github-api.kohsuke.org/</url>
   <description>GitHub API for Java</description>

src/main/java/org/kohsuke/github/GHRepositoryBuilder.java

@@ -167,7 +167,7 @@ public S private_(boolean enabled) throws IOException {
      */
     public S visibility(final Visibility visibility) throws IOException {
         requester.withPreview(NEBULA);
-        return with("visibility", visibility);
+        return with("visibility", visibility.toString());
     }
 
     /**