Fixed - was not working for variables in last line of heredoc.

It seems that heredoc() method excludes terminator line, so no need to pop()
https://metacpan.org/pod/PPI::Token::HereDoc#heredoc

added test for the bug, also added additional test to extract_variables (just
to make sure our regexp deals with newlines right)
(GH-14) Add test for the double-quoted heredocs.
(GH-14) Fix inconsistency in PPI::Token::HereDoc->heredoc().

Due to a bug in PPI::Token::Heredoc, $token->heredoc() does not include the
terminator line when the terminator is a simple bareword, but it includes the
terminator line in the other cases (single/double quoted , commands, escaped).

I wrote a patch to resolve this in PPI (see
https://github.com/adamkennedy/PPI/pull/71), but in the meantime this addresses
the inconsistency.