Add support of uploading media

Returns "Not enough permissions to access media resource".
I hope LinkedIn will resolve it eventually.

Author: zoonman

README.md

@@ -234,6 +234,16 @@ Change default API root
 $client->setApiRoot('https://api.linkedin.com/v2/');
 ```
 
+Try to upload image to LinkedIn. See [Rich Media Shares]()https://developer.linkedin.com/docs/guide/v2/shares/rich-media-shares#upload)
+(returns "Not enough permissions to access media resource" for me). 
+I assume you have to be LinkedIn partner or something like that.
+
+```php
+$filename = '/path/to/image.jpg';
+$client->setApiRoot('https://api.linkedin.com/');
+$mp = $client->upload($filename);
+```
+
 ## Contributing
 
 Please, open PR with your changes linked to an GitHub issue.

examples/demo.jpg

@@ -105,6 +105,13 @@
             );
             pp($companyShare);
 
+            /*
+            // Returns {"serviceErrorCode":100,"message":"Not enough permissions to access media resource","status":403}
+            // You have to be whitelisted or so by LinkedIn
+            $filename = './demo.jpg';
+            $client->setApiRoot('https://api.linkedin.com/');
+            $mp = $client->upload($filename);
+            */
         } catch (\LinkedIn\Exception $exception) {
             // in case of failure, provide with details
             pp($exception);

examples/index.php

@@ -279,6 +279,7 @@ public function getAccessToken($code = '')
                 'headers' => [
                     'Content-Type' => 'application/json',
                     'x-li-format' => 'json',
+                    'Connection' => 'Keep-Alive'
                 ],
             ]);
             try {
@@ -532,6 +533,42 @@ public function post($endpoint, array $params = [])
         return $this->api($endpoint, $params, Method::POST);
     }
 
+    /**
+     * @param $path
+     * @return array
+     * @throws Exception
+     */
+    public function upload($path)
+    {
+        $headers = $this->getApiHeaders();
+        unset($headers['Content-Type']);
+        //$headers = [];
+        if ($this->isUsingTokenParam()) {
+            //
+        } else {
+            $headers['Authorization'] = 'Bearer ' . $this->accessToken->getToken();
+        }
+        $guzzle = new GuzzleClient([
+            'base_uri' => $this->getApiRoot()
+        ]);
+        $options = [
+            'multipart' => [
+                [
+                    'name' => basename($path),
+                    'filename' => basename($path),
+                    'contents' => fopen($path, 'r')
+                ]
+            ],
+            'headers' => $headers,
+        ];
+        try {
+            $response = $guzzle->request(Method::POST, 'media/upload', $options);
+        } catch (RequestException $requestException) {
+            throw Exception::fromRequestException($requestException);
+        }
+        return self::responseToArray($response);
+    }
+
     /**
      * @param array $params
      * @param string $method