fix ExtensionInstaller run on first install

Author: mvorisek

program/include/rcmail_oauth.php

@@ -200,10 +200,29 @@ public function __construct($options = [])
         }
 
         // prepare a http client with the correct options
-        $this->http_client = $this->rcmail->get_http_client((array) $options['http_options'] + [
-            'timeout' => $this->options['timeout'],
-            'verify' => $this->options['verify_peer'],
-        ]);
+        if (class_exists(HttpClient::class)) { // class is not autoloaded in extension installer composer plugin on the first composer install
+            /*
+             * In rcube.php line 277:
+             *
+             *   [Error] Class 'GuzzleHttp\Client' not found
+
+             * Exception trace:
+             *   at program\lib\Roundcube\rcube.php:277
+             *  rcube->get_http_client() at program\include\rcmail_oauth.php:205
+             *  rcmail_oauth->__construct() at program\include\rcmail_oauth.php:104
+             *  rcmail_oauth::get_instance() at program\include\rcmail.php:150
+             *  rcmail->startup() at program\include\rcmail.php:88
+             *  rcmail::get_instance() at program\include\clisetup.php:30
+             *  include_once() at vendor\roundcube\plugin-installer\src\Roundcube\Composer\ExtensionInstaller.php:49
+             *  Roundcube\Composer\ExtensionInstaller->install() at composer.phar/src/Composer/Installer/InstallationManager.php:463
+             *  Composer\Installer\InstallationManager->install() at composer.phar/src/Composer/Installer/InstallationManager.php:378
+             */
+
+            $this->http_client = $this->rcmail->get_http_client((array) $options['http_options'] + [
+                'timeout' => $this->options['timeout'],
+                'verify' => $this->options['verify_peer'],
+            ]);
+        }
     }
 
     /**