Removed isset check for non-existant property

Looks like a left over from the original eloquent model.

src/Model.php

@@ -909,7 +909,7 @@ public function offsetUnset($offset)
      */
     public function __isset($key)
     {
-        return (isset($this->attributes[$key]) || isset($this->relations[$key])) ||
+        return isset($this->attributes[$key]) ||
                 ($this->hasGetMutator($key) && ! is_null($this->getAttributeValue($key)));
     }