Fix: Ensure correct hexadecimal representation for .toString('hex') and .toString(16) methods

[nnnoel]

Fixes #60

Root Cause

The original implementation of the toString method was designed to only handle numerical base representations. When a string-based representation like 'hex' was passed to the method, it was not recognized or processed, and the method defaulted to returning the decimal string representation of the number (base = 10)

Summary:

• Enhanced the toString method to correctly handle string-based base representations, ensuring that .toString('hex') and .toString(16) return the expected hexadecimal format of the number.
• Added a test case to validate the correct conversion of numbers to hexadecimal.

[mrousavy]

Hey, thanks for your PR this looks good!