make normalizeDate more forgiving (#4101)

also parse two-digit days, e.g. "April 20, 2017"

src/Text/Pandoc/Shared.hs

@@ -291,7 +291,7 @@ normalizeDate s = fmap (formatTime defaultTimeLocale "%F")
              parseTime defaultTimeLocale
 #endif
         formats = ["%x","%m/%d/%Y", "%D","%F", "%d %b %Y",
-                    "%d %B %Y", "%b. %d, %Y", "%B %d, %Y",
+                    "%e %B %Y", "%b. %e, %Y", "%B %e, %Y",
                     "%Y%m%d", "%Y%m", "%Y"]
 
 --