Revert "(GH-14) Fix inconsistency in PPI::Token::HereDoc->heredoc()."

This reverts commit 49b2c9b20be10f91c46c3ace145fc6a4a1b15b25.

Upon further investigation, I found that the issue is not in PPI but in the
test cases in this module. I will commit a fix for those separately, but I am
reverting the PPI workaround and I have cancelled the PPI pull request
accordingly.
(GH-14) Fixed here-doc terminators.

The closing terminator should not include quotes/backticks/etc, or it will not
be detected as a valid closing terminator and be included in the heredoc()
output instead, which is the real cause of the issues we've seen in GH-14.