make sequence accept all kinds of json

bigcouch uses arrays for sequence:
http://mail-archives.apache.org/mod_mbox/couchdb-user/201301.mbox/%3CCABvT1DEGDCJyvEuFsNJZbXi_NJwJna8NHDnQvpr2h532s0V18g@mail.gmail.com%3E

So we should not expect them to be integers. Still have to catch the empty file situation in a meaningful way though.