Update Client.php

For correct work of posting shares, I made some changes. Now it's possible to use GET data and POST data in single request. And now it's posible to send XML POST.

POST data sending as 4 parameter in `fetch` method.

Worked example from real project:
```php
$client = new \Splash\LinkedIn\Client($apiKey, $secretKey);
$client->setAccessToken($token);
$data = "
  <share>
	<content>
		<title>This is title</title>
		<description>This is body</description>
		<submitted-url>http://someurl.com/</submitted-url>
		<submitted-image-url>http://lorempixel.com/400/400/nightlife/</submitted-image-url>
	</content>
	<visibility>
		<code>anyone</code>
	</visibility>
  </share>";
$response = $client->fetch('/v1/people/~/shares', array(), 'POST', $data);
```