#1309, #1451 Fixes missing title and cover for spotify playlists by adding user-agent

[mbaumeler]

Spotify returns a bad request when no user-agent is given.

Simple way to reproduce:

<?php
// demo code to reproduce:
$headers = stream_context_create(array('http'=>array('method'=>'GET', 'header'=>'user-agent:Phoniebox')));
$url = "https://open.spotify.com/oembed?url=spotify:playlist:4wFsylSpjoor3HXkle80sa&format=json";

echo "This will work:\n";
echo file_get_contents($url, false, $headers);

echo "\n\nThis will fail (400 Bad Request):\n";
echo file_get_contents($url);
?>

Therefore I added a simple user-agent to all spotify requests. It is similar and inspired by the solution of @acidburn78 but without adding a new dependency.