Uncomment tests for custom walkers, and fix the failures.

Some of the tests for custom walkers were accidentally commented out,
masking some issues with the implementation. The problem was that after a
function has been bound using Function.bind(), its 'this' parameter cannot
be re-bound using 'Function.call'.
Merge pull request #88 from pdubroy/fix-custom-walkers-test

Uncomment tests for custom walkers, and fix the failures.