SF #847346: add test_fix_sentence_endings() to ensure that sentence

SF #847346: add test_fix_sentence_endings() to ensure that sentence
endings get fixed even if the input is too short to wrap.  Also throw
in a bunch of other tests for this rather tricky and under-tested
feature.

In WrapTestCase.setUp(), don't bother setting fix_sentence_endings=True
for self.wrapper -- just use a separate wrapper in the two tests that
care about it, namely test_whitespace() and test_fix_sentence_endings().