Buttons that creates newline in WikiEditor doesn't preserve existing lines

Steps to reproduce:

• Create a new page in WikiEditor with highlight disabled
• Type "a", enter to create a new line and "b"
• Place the cursor next to "a" and insert an horizontal rule through the button

Expected result:

• The content should be:

  a
  ----
  
  b
  

Obtained result:

• The content is:

  a
  ----
  b
  

The newline between a and b is overridden by the one created by the new horizontal rule.