feat: add content-type header

src/client.ts

@@ -102,11 +102,12 @@ export class Client {
     // cut trailing slash if needed
     this.url = this.url.replace(/\/$/, '');
 
+    this.headers = {
+      'Content-Type': 'application/json',
+    };
+
     if (typeof version === 'string') {
-      // set header to tell proxy to select the correct backend
-      this.headers = {
-        'X-StApps-Version': this.version,
-      };
+      this.headers['X-StApps-Version'] = this.version;
     }
   }