Change conditional that checks if val exists

If val was the boolean false, it would skip this step. This is why keys could not have the value false
Add more detailed tests for set and get
Check number of arguments instead of val existence
Add more get/set tests with 'undefined' and 'null'
Verify key type instead of value

This is to make the code cleaner and easier to understand
Merge pull request #3 from yola/enable_boolean_values_for_keys

Enable boolean values for keys