Reinstate inadvertently removed protected method in StaxEventItemWriter

[fmbenhassine]

#5177 has fixed #5098, but introduced a breaking change by removing a protected method: StaxEventItemWriter#endDocument. This issue is to restore that method.

@banseok1216 As you authored that PR, I am CCing you here. If you can put back that method, I would appreciate your help. Otherwise, I will take of that myself. Thank you for your help.

[banseok1216]

Sorry, I missed that StaxEventItemWriter#endDocument is a protected extension point, which caused the problem. 😭

[fmbenhassine]

No problem at all! It's my bad I didn't catch that on review. Thank you for the quick PR 🙏