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
ensure dissoc preserves meta
don't replace keys

fix #3

when associng on a key that's already present, the new key could still have a
different meta value than the existing key. In this case, we want to keep the
existing key and its meta value
Merge branch 'preserve-meta' into new-master
bump test
shorten test
dummy create methods

make the clojure compiler happy
make EMPTY public