Missing header in `/user/installations` requests

[vovayatsyuk]

These two methods need a application/vnd.github.machine-man-preview+json header: https://github.com/KnpLabs/php-github-api/blob/master/lib/Github/Api/CurrentUser.php#L187-L206

Here is a new requirement:
https://developer.github.com/v3/apps/installations/#list-app-installations-accessible-to-the-user-access-token
https://developer.github.com/v3/apps/installations/#list-repositories-accessible-to-the-user-access-token

Note: To access the API with your GitHub App, you must provide a custom media type in the Accept Header for your requests.

application/vnd.github.machine-man-preview+json

I can create a PR and move the methods to Apps.php which already has a method to add required header, or I can copy AcceptHeaderTrait to the CurrentUser class.

How do you want to fix it?

[acrobat]

@vovayatsyuk Can you add the AcceptHeaderTrait to the CurrentUser api class and set the acceptHeader property in each method. See the proctection api forexample.

php-github-api/lib/Github/Api/Repository/Protection.php

Lines 35 to 41 in 0ce1f09

	public function show($username, $repository, $branch)
	{
	// Preview endpoint
	$this->acceptHeaderValue = 'application/vnd.github.luke-cage-preview+json';
	return $this->get('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/branches/'.rawurlencode($branch).'/protection');
	}