add unit tests to validate issue #97
fix Constraint::getErrors implementation

- remove presentation logic from implementation and dont filter returned errors by array_unique, business logic requires getErrors to return all existing errors, even if it contains duplicates
- make Validator::getErrors() return the same filtered result as before and apply array_unique when setting errors on Validator instance
add unit tests to validate issue #76
Merge pull request #98 from alexmmm/fix-constraint-errors-getter

Fix constraint errors getter