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 +160,27 @@ 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);
+    }
+
+    // NOTE: This does NOT work, even when using CURL!
+    // I suspect it's a bug in the API itself...
+    // I should raise a ticket with Gibhub...
+    public void deleteFile(String fileName) throws IOException {
+        LinkedHashMap<String,Object> files = new LinkedHashMap<String, Object>();
+        files.put(fileName, Collections.singletonMap("filename", null));
+        edit("files",files);
+    }
+
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;