[IfNode] Accept and evaluate a Bool value as a valid expression

Sources/IfTag.swift

@@ -63,6 +63,8 @@ public class IfNode : NodeType {
       truthy = !result.isEmpty
     } else if let result = result as? [String:Any] {
       truthy = !result.isEmpty
+    } else if let result = result as? Bool {
+      truthy = result
     } else if result != nil {
       truthy = true
     }