Fix mocking Previews

Author: seregamorph

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

@@ -7,7 +7,7 @@
  *
  * @author Kohsuke Kawaguchi
  */
-enum Previews {
+public enum Previews {
 
     /**
      * Check-runs and check-suites

src/test/java/org/kohsuke/github/GHPullRequestMockTest.java

@@ -0,0 +1,21 @@
+package org.kohsuke.github;
+
+import org.junit.Test;
+
+import java.io.IOException;
+
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class GHPullRequestMockTest {
+
+    @Test
+    public void shouldMockGHPullRequest() throws IOException {
+        GHPullRequest pullRequest = mock(GHPullRequest.class);
+        when(pullRequest.isDraft()).thenReturn(true);
+
+        assertTrue("Mock should return true", pullRequest.isDraft());
+    }
+
+}