MINOR: fix Email class modifying SSViewer.source_file_comments config val

The Email class was updating the SSViewer.source_file_comments value and not
resetting it. If someone had this value set to true in their system and then
used the Email class, it seems there would be an unintended side-effect of
having the source file comments turned off.
Merge pull request #2133 from jthomerson/pulls/fix_testcase_comments_setting

MINOR: fix Email class modifying SSViewer.source_file_comments config val