Fix situation where ABSTRACT comment is empty

Ok, second attempt.

If you have the following line:
    # ABSTRACT:
Then you currently get the next line (following the ABSTRACT comment)
as the abstract.
I think the intention of the code was to not match at this point,
so the abstract would be extracted from the pod

The regexp is now:

    /^\s*#+\s*ABSTRACT:[ \t]*(\S.*)$/m

Need to have the [ \t], rather than

    ABSTRACT:\s*

Because the \s* was slurping up the newline, causing the rest of the previous
regexp to grab the next line. I'm also assuming that DZ expects a single
line abstract comment, and isn't expected to support:

    # ABSTRACT: this is the start of a really long abstract
    #           that runs across multiple lines

Added a testcase, which failed without the change, and passes with the change.
The rest of the testsuite still passes, which is an improvement on my
previous pull request :-)

I'm not at all familiar with the internals of DZ, so suspect that I didn't
write the test in "the right way", but it works.
test for handling of empty ABSTRACT