Comparison expressions need to traverse over their own _value property

Comparisons where not traversing their _value property which was causing issues when comparing another expression.

src/Database/Expression/Comparison.php

@@ -119,6 +119,17 @@ public function sql(ValueBinder $generator) {
 		return sprintf($template, $this->_field, $this->_conjunction, $value);
 	}
 
+/**
+ * {@inheritDoc}
+ *
+ */
+	public function traverse(callable $callable) {
+		if ($this->_value instanceof ExpressionInterface) {
+			$callable($this->_value);
+			$this->_value->traverse($callable);
+		}
+	}
+
 /**
  * Returns a template and a placeholder for the value after registering it
  * with the placeholder $generator