See the addheader method of the HTTPMessage class in httplib.py

    def addheader(self, key, value):
        """Add header for field key handling repeats."""
        prev = self.dict.get(key)
        if prev is None:
            self.dict[key] = value
        else:
            combined = ", ".join((prev, value))
            self.dict[key] = combined


also see the original tracker entry where this fix was first discussed & implemented

https://sourceforge.net/tracker/index.php?func=detail&aid=432621&group_id=5470&atid=105470