FIX Prevent undefined index notice when trying to determine HTTP… (#440)

FIX Prevent undefined index notice when trying to determine HTTP_HOST during dev/build

src/Model/Subsite.php

@@ -870,7 +870,7 @@ class Subsite extends DataObject
         }
 
         // If there are no objects, default to the current hostname
-        return $_SERVER['HTTP_HOST'];
+        return Director::host();
     }
 
     /**

src/Model/SubsiteDomain.php

@@ -185,7 +185,7 @@ class SubsiteDomain extends DataObject
      */
     public function getSubstitutedDomain()
     {
-        $currentHost = $_SERVER['HTTP_HOST'];
+        $currentHost = Director::host();
 
         // If there are wildcards in the primary domain (not recommended), make some
         // educated guesses about what to replace them with: