Add possibility to get headers when send push

[Stafox]

Add possibility to get headers when send push to be able to fetch X-Push-Status-Id

[montymxb]

@Stafox Took a look at this locally to verify everything. X-Parse-Push-Status-Id is present and accounted for in the return value. There is one issue I found regarding a missing parameter in the final return call in ParsePush::send:

return ParseClient::_request(
            'POST',
            'push',
            null,
            json_encode(ParseClient::_encode($data, true)),
            $useMasterKey,
            'application/json',
            true
        );

Directly after $useMasterKey and before the content type there is another parameter called$appRequest. You should update your call to ParseClient::_request to account for that parameter as follows.

return ParseClient::_request(
            'POST',
            'push',
            null,
            json_encode(ParseClient::_encode($data, true)),
            $useMasterKey,
            false,    // app request parameter
            'application/json',
            true
        );

You can simply use false, which is the default (the parameter is only ever used in ParseApp, which is somewhat vestigial at this point to be honest).

Once you make that change I'll take another look. And thanks for submitting this PR in the first place by the way 👍 .

[Stafox]

@montymxb Wow, you're right. I just missed it:( Fixed.

X-Parse-Push-Status-Id is present and accounted for in the return value

Yes, returned array will contain _headers key with headers, so you will able to get push status id by ```

$response = ParsePush::send( [
      'channels' => [''],
      'data'     => ['alert' => 'sample message'],
 ]);

$pushStatusId = $response['_headers']['X-Push-Status-Id'];

[montymxb]

Looks good