fix offset bug

toArray is called on UnrolledChunkedSeq, not on the enclosing CardN
class. As a result, ArrayChunk gets an array with offset elements
chopped off *and* an offset, so offset is doubly applied.

Passing 0 instead of offset to ArrayChunk solves the problem and
restores correct behavior.
Merge pull request #1 from MichaelBlume/fix-offset

fix offset bug