click through URLs in CiviMail are displayed correctly if link text contains "http:"

Click through URLs in CiviMail are displayed incorrectly if link text contains "http:"

To recreate this on the demo, send a mailing with URL cllck-through tracking enabled and the following lines in the body of the HTML message:

1) Click here to see a great tool <a href="http://www.civicrm.org">www.civicrm.org</a>
2) Click here to see a great tool <a href="http://www.civicrm.org">http://www.civicrm.org</a>
3) Click <a href="http://www.civicrm.org">here</a> to see a great tool.

Observe that in the delivered message the click-through addresses are correct for all links, and the link text for lines 1 and 3 remain as in the original file, but for line 2 the link text is replaced with something like
"http://www.homesite.org/sites/all/modules/civicrm/extern/url.php?u=21&qid=7"

Desired behaviour is to allow "http://www.civicrm.org" as unaltered link text.