Fresh start. Added first test testZeroConvertsToEmptyString.
Fixed a typo.
Added test for number 1.
Added test to convert number 5.
Refactoring duplicate code in tests.
Added test for number 10.
Refactoring code duplication in implementation (if statements to enumeration).
Added test for number 2 (recursion).
Added tests for numbers 3 and 4.
Added tests for 6, 7, 8, 9, 50, 100, 500, 1000.
Added tests for 40, 90, 400, 900 and two more complex numbers.