Comparison expressions need to traverse over their own _value property

Comparisons were not traversing their `_value` property which was causing issues when comparing another expression.

Especially prevalent when using SqlServer since it requires transforming of function expressions. An example:

```
$age = $query->func()->dateDiff([
			'Jobs.date_queued' => 'literal',
			$query->func()->now('date'),
		]);
$olderThan30Days = $query->newExpr()->lte(30, $age); //Needs to be this way round as Comparison only treats $value as an expression
```

This was not traversing into the DateDiff, which was causing the `CURRENT_DATE` to not be transformed into the equivalent TSQL statement.