Add ability to unset the milestone of an issue

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

@@ -228,6 +228,13 @@ private void edit(String key, Object value) throws IOException {
         root.createRequest().with(key, value).method("PATCH").withUrlPath(getApiRoute()).send();
     }
 
+    /**
+     * Identical to edit(), but allows null for the value.
+     */
+    private void editNullable(String key, Object value) throws IOException {
+        root.createRequest().withNullable(key, value).method("PATCH").withUrlPath(getApiRoute()).send();
+    }
+
     private void editIssue(String key, Object value) throws IOException {
         root.createRequest().with(key, value).method("PATCH").withUrlPath(getIssuesApiRoute()).send();
     }
@@ -277,15 +284,19 @@ public void setBody(String body) throws IOException {
     }
 
     /**
-     * Sets milestone.
+     * Sets the milestone for this issue.
      *
      * @param milestone
-     *            the milestone
+     *            The milestone to assign this issue to. Use null to remove the milestone for this issue.
      * @throws IOException
-     *             the io exception
+     *             The io exception
      */
     public void setMilestone(GHMilestone milestone) throws IOException {
-        edit("milestone", milestone.getNumber());
+        if (milestone == null) {
+            editNullable("milestone", null);
+        } else {
+            edit("milestone", milestone.getNumber());
+        }
     }
 
     /**

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

@@ -7,8 +7,6 @@
 import java.io.IOException;
 import java.util.Date;
 
-import static org.junit.Assert.assertEquals;
-
 /**
  * @author Martin van Zijl
  */
@@ -23,7 +21,8 @@ public void cleanUp() throws Exception {
         }
 
         for (GHMilestone milestone : getRepository(getGitHubBeforeAfter()).listMilestones(GHIssueState.ALL)) {
-            if ("Original Title".equals(milestone.getTitle()) || "Updated Title".equals(milestone.getTitle())) {
+            if ("Original Title".equals(milestone.getTitle()) || "Updated Title".equals(milestone.getTitle())
+                    || "Unset Test Milestone".equals(milestone.getTitle())) {
                 milestone.delete();
             }
         }
@@ -54,6 +53,23 @@ public void testUpdateMilestone() throws Exception {
         assertEquals(OUTPUT_DUE_DATE, milestone.getDueOn());
     }
 
+    @Test
+    public void testUnsetMilestone() throws IOException {
+        GHRepository repo = getRepository();
+        GHMilestone milestone = repo.createMilestone("Unset Test Milestone", "For testUnsetMilestone");
+        GHIssue issue = repo.createIssue("Issue for testUnsetMilestone").create();
+
+        // set the milestone
+        issue.setMilestone(milestone);
+        issue = repo.getIssue(issue.getNumber()); // force reload
+        assertEquals(milestone.getNumber(), issue.getMilestone().getNumber());
+
+        // remove the milestone
+        issue.setMilestone(null);
+        issue = repo.getIssue(issue.getNumber()); // force reload
+        assertEquals(null, issue.getMilestone());
+    }
+
     protected GHRepository getRepository() throws IOException {
         return getRepository(gitHub);
     }

src/test/resources/org/kohsuke/github/GHMilestoneTest/wiremock/testUnsetMilestone/__files/orgs_github-api-test-org-2.json

@@ -0,0 +1,41 @@
+{
+  "login": "github-api-test-org",
+  "id": 7544739,
+  "node_id": "MDEyOk9yZ2FuaXphdGlvbjc1NDQ3Mzk=",
+  "url": "https://api.github.com/orgs/github-api-test-org",
+  "repos_url": "https://api.github.com/orgs/github-api-test-org/repos",
+  "events_url": "https://api.github.com/orgs/github-api-test-org/events",
+  "hooks_url": "https://api.github.com/orgs/github-api-test-org/hooks",
+  "issues_url": "https://api.github.com/orgs/github-api-test-org/issues",
+  "members_url": "https://api.github.com/orgs/github-api-test-org/members{/member}",
+  "public_members_url": "https://api.github.com/orgs/github-api-test-org/public_members{/member}",
+  "avatar_url": "https://avatars3.githubusercontent.com/u/7544739?v=4",
+  "description": null,
+  "is_verified": false,
+  "has_organization_projects": true,
+  "has_repository_projects": true,
+  "public_repos": 25,
+  "public_gists": 0,
+  "followers": 0,
+  "following": 0,
+  "html_url": "https://github.com/github-api-test-org",
+  "created_at": "2014-05-10T19:39:11Z",
+  "updated_at": "2015-04-20T00:42:30Z",
+  "type": "Organization",
+  "total_private_repos": 0,
+  "owned_private_repos": 0,
+  "private_gists": 0,
+  "disk_usage": 147,
+  "collaborators": 0,
+  "billing_email": "kk@kohsuke.org",
+  "default_repository_permission": "none",
+  "members_can_create_repositories": false,
+  "two_factor_requirement_enabled": false,
+  "plan": {
+    "name": "free",
+    "space": 976562499,
+    "private_repos": 0,
+    "filled_seats": 15,
+    "seats": 0
+  }
+}