Update Server.kt

This string allows you, for example, upload your app to Heroku while you DB is in MongoDB Atlas. In this case CORS blocks your POST requests. See https://youtrack.jetbrains.com/issue/KTOR-4008/POST-request-to-database-returns-403-error#focus=Comments-27-6434385.0-0

src/jvmMain/kotlin/Server.kt

@@ -30,6 +30,7 @@ fun main() {
             json()
         }
         install(CORS) {
+            allowHeader(HttpHeaders.ContentType)
             allowMethod(HttpMethod.Get)
             allowMethod(HttpMethod.Post)
             allowMethod(HttpMethod.Delete)
@@ -65,4 +66,4 @@ fun main() {
             }
         }
     }.start(wait = true)
-}
+}