Fix the error in html email when pear mail_mime is missing

When pear package mail_mime is missing, attempt to send html email throws error:
mail() expects parameter 3 to be string, array given.  This is because the code
doesn't unset the html part of the body when falling back to text email

Change-Id: Ie8c73a481cb9003a0b43893cf4751475bdf5c690
Merge "Fix the error in html email when pear mail_mime is missing"