Backport the enhancements to Pivot to 5.5 since it is LTS

[jimmypuckett]

• Laravel Version: 5.5.x (LTS)
• PHP Version: >= 7
• Database Driver & Version: mysql

Description:

We are having issues when a pivot model is used standalone to work with the pivot data. There is logic in the model that issues that it is only being used as a pivot. For example...

/**
  * Get the name of the "updated at" column.
  *
  * @return string
  */
  public function getUpdatedAtColumn()
  {
      return $this->pivotParent->getUpdatedAtColumn();
  }

However, as of 5.6, this bug was fixed..

 /**
    * Get the name of the "updated at" column.
    *
    * @return string
    */
    public function getUpdatedAtColumn()
    {
        return ($this->pivotParent)
                        ? $this->pivotParent->getUpdatedAtColumn()
                        : parent::getUpdatedAtColumn();
    }

Since 5.5 is LTS, can we get this model back ported into a 5.5 tag?

Steps To Reproduce:

1. Make a pivot model
2. Work directly with the model

[staudenmeir]

Will be fixed in the next release (#25746).