fix indentation of void elements with end slash

[adamjimenez]

Fixes an indentation bug with the beautify extension and void elements with end slassh.
e.g.

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Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts

[nightwing]

Looks good, but this breaks an existing test case.
Could you check that, and also please add a test for the new behavior too.

[nightwing]

see #5651 (comment)