This fix handles the case where an In-Reply-To mail header
can contain multiple Message-IDs. We use this header to
try look up an EmailLog record to find the post to reply
to in the group email inbox flow.
Since the case where multiple In-Reply-To Message-IDs is
rare (we've only seen a couple of instances of this causing
errors in the wild), we are just going to use the first one
in the array.
Also, Discourse does not support replying to multiple posts
at once, so it doesn't really make sense to use multiple
In-Reply-To Message-IDs anyway.
