Add BEST_MATCH and expand test

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

@@ -611,6 +611,23 @@ class GitHubRequest {
             return with(key, value);
         }
 
+        /**
+         * Unset the key field
+         *
+         * @param key
+         *            the key
+         * @return the request builder
+         */
+        public B unset(String key) {
+            for (int index = 0; index < args.size(); index++) {
+                if (args.get(index).key.equals(key)) {
+                    args.remove(index);
+                    break;
+                }
+            }
+            return (B) this;
+        }
+
         /**
          * Method requester.
          *