Fix callable string calls global function

This fixes a bug where a value, such as `exec`, is technically callable
but we don’t expect it to be actually callable as it’s likely to be a
value of a model.

src/Entity.php

@@ -40,7 +40,7 @@ abstract class Entity implements Arrayable
       $value = $this->object->{$name};
     }
 
-    $value = is_callable($value) ? $value() : $value;
+    $value = !is_string($value) && is_callable($value) ? $value() : $value;
 
     $this->exposes[$name] = $value;
   }