correctly estimate the free space on the compressed page by
page_zip_available(..., create=TRUE). This was reported as Issue #231.

btr_cur_update_alloc_zip(): Add the parameter ibool create and pass it
to page_zip_available(). The parameter was previously passed as 0.

btr_cur_optimistic_update(): Pass create=TRUE to btr_cur_update_alloc_zip().

rb://120 approved by Heikki Tuuri