PhabricatorMailImplementationPHPMailerLiteAdapter ignores parameter in setIsHTML function.

Summary: Fix PhabricatorMailImplementationPHPMailerLiteAdapter to actually use
given parameter.

Test Plan: Use setIsHTML with false as parameter, sent mail should be in plain
text.

Reviewers: jungejason

Reviewed By: jungejason

CC: aran, jungejason, epriestley

Differential Revision: 1001