Fields "To" and "Subject" are visible for user during the preview of Private Message

Steps to reproduce:
1. Login to Forum if you were logged out.
2. Click on User Menu - > "Messages"
3. Click on "Compose".
4. Fill out fields "To" and "Subject" with following test data:

To - test
Subject - test
Message content - test

5. Click on "Preview" button.

Actual results:
PM Preview window with "To", "Subject", "Message content" fields filled out with test data opens.

Expected results:
Only "Message content" field should be shown for user during the preview of PM.