Implement AutoCloseable to close the underlying client. (#520)

src/main/java/org/gitlab4j/api/GitLabApi.java

 package org.gitlab4j.api;
 
+import java.io.Closeable;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
@@ -23,7 +24,7 @@ import org.gitlab4j.api.utils.SecretString;
  * This class is provides a simplified interface to a GitLab API server, and divides the API up into
  * a separate API class for each concern.
  */
-public class GitLabApi {
+public class GitLabApi implements AutoCloseable {
 
     private final static Logger LOGGER = Logger.getLogger(GitLabApi.class.getName());
 
@@ -482,6 +483,17 @@ public class GitLabApi {
         return (this);
     }
 
+
+    /**
+     * Close the underlying {@link javax.ws.rs.client.Client} and its associated resources.
+     */
+    @Override
+    public void close() {
+        if (apiClient != null) {
+            apiClient.close();
+        }
+    }
+
     /**
      * Enable the logging of the requests to and the responses from the GitLab server API
      * using the GitLab4J shared Logger instance and Level.FINE as the level.

src/main/java/org/gitlab4j/api/GitLabApiClient.java

@@ -48,7 +48,7 @@ import org.glassfish.jersey.media.multipart.file.FileDataBodyPart;
 /**
  * This class utilizes the Jersey client package to communicate with a GitLab API endpoint.
  */
-public class GitLabApiClient {
+public class GitLabApiClient implements AutoCloseable {
 
     protected static final String PRIVATE_TOKEN_HEADER  = "PRIVATE-TOKEN";
     protected static final String SUDO_HEADER           = "Sudo";
@@ -243,6 +243,16 @@ public class GitLabApiClient {
         clientConfig.register(MultiPartFeature.class);
     }
 
+    /**
+     * Close the underlying {@link Client} and its associated resources.
+     */
+    @Override
+    public void close() {
+        if (apiClient != null) {
+            apiClient.close();
+        }
+    }
+
     /**
      * Enable the logging of the requests to and the responses from the GitLab server API.
      *