Add support for editing Gists.

Fixes issue hub4j#466

NOTE: I could not get the deleteFile() method to work. Possibly an
issue with the underlying API itself.

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

@@ -6,6 +6,7 @@
 import java.net.URL;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Map.Entry;
 
@@ -159,6 +160,57 @@ public void delete() throws IOException {
         new Requester(root).method("DELETE").to("/gists/" + id);
     }
 
+    private String getApiRoute() {
+        return "/gists/" + id;
+    }
+
+    private void edit(String key, Object value) throws IOException {
+        new Requester(root)._with(key, value).method("PATCH").to(getApiRoute());
+    }
+
+    public void setDescription(String value) throws IOException {
+        edit("description",value);
+    }
+
+    /**
+     * Adds a file.
+     */
+    public void addFile(String fileName, String content) throws IOException
+    {
+        updateFile(fileName, content);
+    }
+
+//    /**
+//     * Deletes a file.
+//     */
+//    // NOTE: I can't get this to work, even when using "curl" from the command
+//    // line. It may be an issue with the underlying API itself.
+//    public void deleteFile(String fileName) throws IOException {
+//        LinkedHashMap<String,Object> filesMap = new LinkedHashMap<String, Object>();
+//        filesMap.put(fileName, Collections.singletonMap("filename", null));
+//        edit("files",filesMap);
+//    }
+
+    /**
+     * Replaces the content of a file.
+     */
+    public void updateFile(String fileName, String content) throws IOException
+    {
+        LinkedHashMap<String,Object> filesMap = new LinkedHashMap<String, Object>();
+        filesMap.put(fileName, Collections.singletonMap("content", content));
+        edit("files",filesMap);
+    }
+
+    /**
+     * Renames a file.
+     */
+    public void renameFile(String fileName, String newFileName) throws IOException
+    {
+        LinkedHashMap<String,Object> filesMap = new LinkedHashMap<String, Object>();
+        filesMap.put(fileName, Collections.singletonMap("filename", newFileName));
+        edit("files",filesMap);
+    }
+
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;