Conditions for checking duplicate person is not case sensitive

[nus-se-bot]

Describe the bug
User can input John Do and john do (both with the same attributes) and it is accepted by the system as two different persons.

To Reproduce
Steps to reproduce the behavior:

1. Go to command line
2. Type an example of an add person function:

add n/john do p/98765431 e/john@example.com a/311, Clementi Ave 2, #2-25 s/100000 o/banker r/single

add another one with

add n/John Do p/98765431 e/john@example.com a/311, Clementi Ave 2, #2-25 s/100000 o/banker r/single

3. Scroll down to the bottom of the list
4. See that both are added together

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

---

Reported by: @shimeilee
Severity: High

_{[original: nus-cs2103-AY1819S2/pe-dry-run#833]}