Fix two bugs in the search tree iteration code

The first could occur when a node had two records with the same value. In that
case, we'd provide the wrong IP address for the left record to the callback.

The second was a bug with Math::Uint128:

    $num |= $x;

is not the same as

    $num = $num | $x;
Fix method name misspelling
Doc iterate_search_tree fixes in Changes
Merge pull request #8 from maxmind/dave/iterator-ipnum-bug-fix

Dave/iterator ipnum bug fix