[Webkit] Unable to exit from pre block in some cases

[Karen Ananiev]

• clear all contents
  
• change paragraph format to Preformatted
  
• type some text and press Shift+Enter to create new line
  
• switch to the Source (notice that pre block has extra line break) and go back to the wysiwyg mode
  
• put cursor to the empty second line of pre
  
• press Enter to exit from pre block
  

Outcome: We are still into pre block.
Expected: Exit from pre block and create the new paragraph.

The problem is that enterkey plugin appends bogus br element which is converted to the invisible line break.