Pre output formatting is broken

[Garry Yao]

Note this's not a DUP of #3188, where this ticket should deal with the basic <pre> content foramtting.

Reproducing Procedures

1. Open the replace by class example page in FF;
2. Load the following content with 'source' mode;

   <pre>
   first line
   second line
   
   </pre>
   

3. Switch to 'wysiwyg' mode and switch back.
   • Actual Result:

     <pre>
     	first line second line</pre>
     

   • Expected Result:

     <pre>first line
     second line
     
     </pre>
     

[Garry Yao]

Since browser will remove spaces at the front of block, update the expected result accordingly.