Declare equals() as final

[jeremykendall]

Addresses suggestions from conversation in #4.

• Adds final to equals()
• Update tests per suggestions from @mirfilip
  • Rewrite original tests
  • Add tests proving equals() handles problematic values properly
• Add equals() to documentation

[mirfilip]

That's a solid PR 👍

[chippyash]

Good practice. Good pr.

[mnapoli]

Awesome!

[jeremykendall]

Thanks everyone!