Add "User-Agent" Header setter and getter (#1253)

Author: jmini

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

@@ -880,6 +880,24 @@ public void setIgnoreCertificateErrors(boolean ignoreCertificateErrors) {
         apiClient.setIgnoreCertificateErrors(ignoreCertificateErrors);
     }
 
+    /**
+     * Returns the configured custom 'User-Agent' header value
+     *
+     * @return User-Agent header value
+     */
+    public String getUserAgentHeader() {
+        return apiClient.getUserAgentHeader();
+    }
+
+    /**
+     * Configures a custom value for the 'User-Agent' header
+     *
+     * @param userAgentHeader User-Agent header value
+     */
+    public void setUserAgentHeader(String userAgentHeader) {
+        apiClient.setUserAgentHeader(userAgentHeader);
+    }
+
     /**
      * Get the version info for the GitLab server using the GitLab Version API.
      *