#710 - fix embeding images with multiple inline images

[Pyrol]

reposting request: bpali#1

after #580 ($contentId || 'inline') replaces every inline image within an email with the first one found. So fix #580 is invalid for that case.

Also a question if checking disposition is really necessary? If it is then i'd propose code below

$contentMatch = $attachment->contentId == $cid;
if ($attachment->disposition && !empty($attachment->disposition)) {
    $contentMatch &= 'inline' == \mb_strtolower((string) $attachment->disposition);
}

if ($contentMatch) {
//convert to base64