Make definition of HTTP headers always as arrays and not strings

To ease further HTTP headers manipulation with future HTTPS proxy (see #249) and `GOOGLE_ABUSE_EXEMPTION` cookie.

Verifying current `getContextFromOpts` correctness:

```php
<?php

$headers = 'test';

foreach ($headers as $headerIndex => $header) {
    echo "header: $header\n";
}
```
```
PHP Warning:  foreach() argument must be of type array|object, string given in /home/benjamin/test.php on line 5
```

commentThreads.php

@@ -89,7 +89,7 @@ function getAPI($videoId, $commentId, $order, $continuationToken, $simulatedCont
         $opts = [
             'http' => [
                 'method' => 'POST',
-                'header' => 'Content-Type: application/json',
+                'header' => ['Content-Type: application/json'],
                 'content' => json_encode($rawData),
             ]
         ];

liveChats.php

@@ -66,7 +66,7 @@ function getItem($id)
 
         $opts = [
             'http' => [
-                'header' => 'Content-Type: application/json',
+                'header' => ['Content-Type: application/json'],
                 'method'  => 'POST',
                 'content' => json_encode($rawData),
             ]
@@ -106,7 +106,7 @@ function getItem($id)
             $participants = [];
             $opts = [
                 'http' => [
-                    'header' => 'User-Agent: ' . USER_AGENT,
+                    'header' => ['User-Agent: ' . USER_AGENT],
                 ]
             ];
 

playlistItems.php

@@ -48,7 +48,7 @@ function getAPI($playlistId, $continuationToken)
             'continuation' => $continuationToken
         ];
         $http['method'] = 'POST';
-        $http['header'] = 'Content-Type: application/json';
+        $http['header'] = ['Content-Type: application/json'];
         $http['content'] = json_encode($rawData);
     } else {
         $url = "https://www.youtube.com/playlist?list=$playlistId";

search.php

@@ -123,7 +123,7 @@ function getAPI($id, $order, $continuationToken)
         $opts = [
                'http' => [
                    'method' => 'POST',
-                   'header' => 'Content-Type: application/json',
+                   'header' => ['Content-Type: application/json'],
                    'content' => json_encode($rawData),
                ]
         ];
@@ -162,7 +162,7 @@ function getAPI($id, $order, $continuationToken)
         $opts = [
             'http' => [
                 'method' => 'POST',
-                'header' => 'Content-Type: application/json',
+                'header' => ['Content-Type: application/json'],
                 'content' => json_encode($rawData),
             ]
         ];