fix for doc_underline: a string containing two underscores

has been recognized as string to be underlined.

lib/python/StructuredText/ClassicDocumentClass.py

@@ -632,7 +632,7 @@ class DocumentClass:
     
     def doc_underline(self,
                       s,
-                      expr=re.compile("\_([%s0-9\s\.,\?\/]+)\_" % letters).search):
+                      expr=re.compile("\s*\_([%s0-9\s\.,\?\/]+)\_" % letters).search):
         
         result = expr(s)
         if result: