feat(validation): add case sensitive option for unique validation rule

[Xstoudi]

Proposed changes

As mentionned in #419, unique validation is working correctly when we talk about emails. This PR adds a caseSensitivity argument to the unique validator to allow to ensure that foo@bar.com !== Foo@Bar.com.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc dead link
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works.
• [] I have added necessary documentation (if appropriate)

Further comments

Please let me know if this PR is a satisfying solution or if it is not. I'll PR the documentation asap.

[coveralls]

Coverage decreased (-78.6%) to 17.891% when pulling 343637c on XavierForks:feature-unique-case into 6cd666d on adonisjs:develop.

[thetutlage]

Tests seems to be failing

[thetutlage]

Fixed in 29f7dfc