fixed nested lists and wrote correct tests

nested lists did not work: after a nested list was over,
a new line was inserted. this leads to a large gap before
the rest of the parent list.

lists are prefixed and suffixed with a single newline,
this is now represented in the tests.

markdownify/__init__.py

@@ -169,7 +169,8 @@ class MarkdownConverter(object):
                 break
             el = el.parent
         if nested:
-            text = '\n' + self.indent(text, 1)
+            # remove trailing newline if nested
+            return '\n' + self.indent(text, 1).rstrip()
         return '\n' + text + '\n'
 
     convert_ul = convert_list