Fix S3 sync issue with keys containing urlencode values

Fixes #749.  This was a regression from the fix for #675
where we use the encoding_type of "url" to workaround
the stdlib xmlparser not handling new lines.

The problem is that pagination in s3 uses the last key name as
the marker, and because the keys are returned urlencoded, we
need to urldecode the keys so botocore sends the correct next
marker.  In the case where urldecoded(key) != key we will incorrectly
sync new files.

Also added an integ test for syncing with '+' chars.

cc @danielgtaylor