Fix API of example code

This briefly tripped me up.
Merge pull request #180 from blittle/master

Fix API of example code
Accept null/undefined as constructor for Collections

ES6 Map and Set allow null/undefined as the constructor argument and return empty Map/Set in those cases. They do not allow false.

Mirroring that behavior actually simplifies a few things as well.
Fix incorrect 'this' in Cursor __iterator
Merge pull request #186 from tgriesser/cursor-context-fix

Fix incorrect 'this' in Cursor __iterator
fix OrderedMap#remove