[5.7] Improve "exists" validation with array values

[staudenmeir]

Fixes return values in tests:
getCount() and getMultiCount() return an integer instead of a boolean.

Fixes duplicate values:
If the user supplies an array with duplicate values, the validation can fail (where it should pass):

$v = new Validator($trans, ['email' => ['foo', 'foo']], ['email' => 'exists:users']);

// Fails if there is only one "foo" entry.
$v->passes();

Fixes duplicate database values:
If the database contains duplicate values, the validation can pass (where it should fail):

$v = new Validator($trans, ['first_name' => ['John', 'Jim']], ['first_name' => 'exists:users']);

// Passes if there is no "Jim" entry but 2+ "John"s entries.
$v->passes();