Make sure to add accept header

Author: Nyholm

lib/Github/Client.php

@@ -158,6 +158,8 @@ public function __construct(HttpClient $httpClient = null)
         $this->addPlugin(new Plugin\HeaderDefaultsPlugin(array(
             'User-Agent'  => 'php-github-api (http://github.com/KnpLabs/php-github-api)',
         )));
+        // Add standard headers.
+        $this->clearHeaders();
     }
 
     /**
@@ -430,6 +432,12 @@ public function setOption($name, $value)
             throw new InvalidArgumentException(sprintf('Undefined option called: "%s"', $name));
         }
 
+        if ($name === 'api_version') {
+            $this->addHeaders([
+                'Accept' => sprintf('application/vnd.github.%s+json', $value),
+            ]);
+        }
+
         $this->options[$name] = $value;
     }