Fix off-by-one error in lastChunkNumber. This bug causes potential dataloss and an uncaught thrown error when appending data (using mode "w+") to an existing GridFS file that currently ends exactly on a chunk boundary: (file.length > 0) &&( file.length % chunkSize == 0 ).  Included test lastChunkNumber_off_by_one.js reproduces the bug for unpatched versions of gridstore.js