fix: set content-type header

Fixes #442

Author: wkillerud

lib/podlet.js

@@ -995,9 +995,8 @@ export default class PodiumPodlet {
                 // set "incoming" on res.locals.podium
                 objobj.set('locals.podium', incoming, res);
 
-                if (res.header) {
-                    res.header('podlet-version', this.version);
-                }
+                res.header('podlet-version', this.version);
+                res.header('content-type', 'text/html');
 
                 res.sendHeaders = () => {
                     res.write('');