Treat Buffers as Arrays when rewriting MongoDB atoms.

mongodb 1.4.0-rc9 has a `binId` field of type Buffer on ObjectIDs, which
caused Meteor to crash when retrieving a document that was made up of
just an EJSON user-defined type. (Which should not necessarily be
supported, but `replaceNames` should probably treat Buffers as Arrays
regardless.)

Fixes #2095.