When a custom key is passed to bson_append_element it gets wrongly appended to the buffer twice (once by bson_append_estart, once by a separate bson_append). Ther data_size calculation is also wrong (need to subtract one element type marker and one for the null byte at the end of the key).

See http://github.com/mikejs/mongo-c-driver/commit/5b311e444f361b0b4828430d3af75513bbf102fe