Use getValue for proxy.auth to cope with legacy configs.

The configuration template had 'proxy.auth = false' listed for
years. Prevent installs from breaking on this on upgrade.

modules/core/www/frontpage_config.php

@@ -67,7 +67,7 @@ if ($config->getBoolean('admin.checkforupdates', true) && $current !== 'master')
         curl_setopt($ch, CURLOPT_USERAGENT, 'SimpleSAMLphp');
         curl_setopt($ch, CURLOPT_TIMEOUT, 2);
         curl_setopt($ch, CURLOPT_PROXY, $config->getString('proxy', null));
-        curl_setopt($ch, CURLOPT_PROXYUSERPWD, $config->getstring('proxy.auth', null));
+        curl_setopt($ch, CURLOPT_PROXYUSERPWD, $config->getValue('proxy.auth', null));
         $response = curl_exec($ch);
 
         if (curl_getinfo($ch, CURLINFO_HTTP_CODE) === 200) {