def itermerged(self):
        """Iterate over all headers, merging duplicate ones together."""
        for key in self:
            val = self._container[key.lower()]
            yield val[0], ', '.join(val[1:])
Iterate over all headers, merging duplicate ones together.
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_vendor/urllib3/_collections.py