Allow custom headers on upload and download

[mlambley]

Backblaze allows custom headers on upload, such as X-Bz-Info-b2-content-disposition which saves a Content-Disposition on the file, allowing you to have your file stored as one name but download as another.

Likewise, on download they allow the b2ContentDisposition header to override the file name on download.

Given the likelihood that other such headers exist, in the functions upload, download, uploadLargeFile/uploadParts please allow an extra option Headers which merges into the default header values if it exists such as

$response = $this->client->guzzleRequest('POST', $uploadUrl, [
    'headers' => array_merge([
        'Authorization' => $this->authToken,
    ], $options['Headers'] ?? [])
]);

Happy to submit a PR if you wish.

[mlambley]

Implemented in 1.5.0