API: try to place source_file_comments after doctype instead of html
MINOR: moved includeDebuggingComments logic into own method to allow separated tests
MINOR: if no doctyle is set, include comments inside <html>
MINOR: if <html> tag is inside a html comment do not add a comment
MINOR: updated and extended Tests
Merge pull request #1793 from Zauberfisch/3.1

API: made more robust SSTemplateParser->includeDebuggingComments()