Case insensitive search filter

[soyuka]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets
License	MIT
Doc PR	dunglas/doc#5

I refactored the code a bit by removing nested conditions, hope that's ok.

[sroze]

LGTM 👍

[dunglas]

Thank you @soyuka

[teohhanhui]

I don't understand why the case-sensitivity should be applied at the filter-level instead of per-property.

It's weird that this uses an instance property ($this->caseSensitive).

[teohhanhui]

Also, what of the collations where the matching is already case-insensitive in the first place (like what is most commonly used in MySQL - utf8_unicode_ci or utf8mb4_unicode_ci)? We might want to add a note somewhere...

[soyuka]

+1 @teohhanhui can you add this to the docs?

About the property we discussed it in the PR on 1.x, because it's reset here and to avoid lots of repetition to pass the flag to the caseWrap function, I think it's better to keep the instance property.